UNIVERSITY  OF  CALIFORNIA. 

aass   A1.U 

x  5^.0 


The  Fire  Assay 

of  Gold,  Silver,  and  Lead  in  Ores  and 
Metallurgical  Products 


by 


Leonard  S.[Austin 

Professor  of  Metallurgy  and  Ore-dressing, 
Michigan  College  of  Mines. 


First  Edition 


1907 

Published  by  the 

Mining  and  Scientific  Press 

San  Francisco 


GENEBAL 


COPYRIGHT,  1907 

BY 
MINING  AND  SCIENTIFIC  PRESS 


TABLE  OF   CONTENTS 


PAGE 

Preface   7 

I.  The  Fire  Assay  of  Ores  or  Furnace  and  Mill   Products 

Containing  Gold,  Silver,  and  Lead 11 

II.    'Sampling  and  Preparation  of  the  Ore  for  Assay 12 

III.  Care  of  the  Assay  Office 15 

IV.  Apparatus    ;r;; 17 

V.     The  Assay  Furnace ." 22 

VI.     Crucibles  and  Scorifiers   30 

VII.     Assay  Balances   32 

VIII.     Fluxes  Used  in  Assaying 40 

IX.     Ores 48 

X.     The  Scorification  Assay  50 

XL     Cupelling   54 

XII.     Parting 56 

XIII.  The  Crucible  Assay  60 

XIV.  Roasting  of  Ores   70 

XV.     Assay  of  Matte   72 

XVI.     High-Grade   Silver-Sulphide   Assay 73 

XVII.     Assay  of  Cyanide  Solutions   74 

XVIII.     Assay  of  BaserBullion  - 76 

XIX.     Assay  of  Silver  Bars  or  Ingots  78 

XX.  Assay  of  Blister    or    Pig-Copper    Containing    Silver    and 

Gold    82 

XXI.     Assay  of  Gold  Bullion   84 

XXII.     Assay  of  Ores  Containing  Metallics  85 

XXIII.     The  Lead  Assay 87 


179709 


PREFACE 

The  following  pages  present  a  system  of  assaying,  intended 
to  cover  determinations  of  the  precious  metals  (silver  and  gold) 
and  of  lead,  according  to  methods  quite  commonly  recognized 
in  the  Rocky  Mountain  States  as  suited  to  ores  and  products 
containing  the  just-named  metals. 

In  reading  books  on  the  subject,  the  beginner  in  assaying  is 
embarrassed  in  deciding  which  one  of  the  various  prescribed 
charges  he  is  to  employ.  It  has,  therefore,  been  the  aim  of  the 
author  in  this  book  to  carry  through  a  single  system,  which,  while 
not  the  only  one,  would  at  least  be  definite  and  clear  to  the  learner. 


AUSTIN'S  FIRE  ASSAY 


I.— THE   FIRE  ASSAY   OF   ORES   OR   FURNACE   AND 

MILL  PRODUCTS  CONTAINING  GOLD, 

SILVER,  AND  LEAD. 

The  fire  assay  is  a  dry  or  fusion  method  for  the  determination 
of  the  metal  sought  with  the  aid  of  heat  and  suitable  fluxes,  and 
of  weighing  it  in  metallic  form.  Thus  if  we  desire  to  determine 
lead  in  an  ore  we  fuse  a  known  weight  of  the  ore  in  a  crucible  to- 
gether with  suitable  fluxes  and  obtain  a  lead  button  which,  upon 
weighing,  will  give  us  the  percentage  of  contained  lead  in  the 
portion  of  ore  so  taken,  called  the  sample.  This  sample  must 
be  a  true  average  of  the  lot  of  ore  whose  value  we  wish  thus  to 
determine. 

The  steps  in  assaying  are: 

1.  Receiving  and  labelling  the  ore. 

2.  Sampling. 

3.  Fluxing,  melting,  and  cupelling. 

4.  Weighing  the  metal  obtained. 

5.  Recording  and  reporting  results. 

Generally  but  one  metal  is  determined  in  an  assay,  except  that 
in  the  case  of  gold  and  silver,  the  two  metals,  recovered  as  one, 
are  subsequently  parted. 

The  work  should  be  systematically  arranged  and  carried  out. 
The  secret  of  doing  a  day's  work  well,  both  in  quantity  and  in 
accuracy,  depends  largely  upon  this. 

Never  report  results  where  there  is  reason  to  believe  that  they 
may  be  incorrect  owing  to  faulty  manipulation,  to  losses,  or  to 
the  mixing  of  samples. 

Single  assays  may  be  made  upon  grab  samples  when  an  ap- 
proximation is  sufficient,  but  for  results  involving  the  purchase 
of  metals  or  ores,  duplicates  should  be  run  which  must  closely 
agree,  and,  if  not,  then  the  work  must  be  repeated  until  they  do 
so. 


IL— SAMPLING    AND    PREPARATION    OF    THE    ORE 

FOR  ASSAY. 

All  metals,  ores,  furnace  and  mill-products,  which  are  to  be 
assayed,  must  first  be  accurately  sampled.  This  is  quite  as  im- 
portant as  accurate  assaying,  for,  if  the  sample  does  not  truly 
represent  the  lot  or  quantity  of  ore  whose  value  is  sought,  the 
assay  of  it  will  be  misleading. 

The  following  instructions  on  sampling  ores  are  here  given, 
since  the  assayer  is  often  called  upon  to  sample  the  ore  which 
he  is  about  to  assay. 

RECEIVING  THE  SAMPLE.  The  ore,  mineral,  or  metallurgical 
product  may  come  as  a  hand-sample,  a  'grab'-sample,  or  as  a 
regular  lot-sample  from  the  sampling  mill.  Qnly  regular  or 
mill-samples  are,  in  general,  assayed  in  duplicate. 

Small  tin  or  graniteware  pans  are  used  to  receive  hand- 
samples,  each  being  placed  in  a  separate  pan,  and  with  a  label, 
upon  which  is  marked  the  name  of  the  owner,  the  name  of  the 
ore,  and  the  lot-number  of  the  sample.  Where  the  assayer  has 
to  number  the  lots  himself,  he  should  also  describe  the  ore  so 
that  it  may  be  distinguished  again  by  the  sender. 

It  is  a  good  practice  to  reserve  one  or  more  of  the  larger  pieces, 
after  having  partly  broken  down  a  sample,  since  in  this  way  we 
may  be  better  able  to  designate  or  otherwise  recognize  the  ore. 

The  hand-sample  may  consist  of  one  or  of  several  lumps,  of  a 
mixture  of  fine  and  of  lumpy  ore,  or  of  a  regularly  ground  sample 
of  ore  ready  for  assaying.  In  the  latter  case,  the  assayer  should 
be  especially  on  his  guard  to  exercise  due  care,  since  another 
portion  of  the  same  'pulp'  can  be  used  as  a  check  on  his  work 
at  another  time  or  by  another  assayer. 

PREPARATION  OF  .THE  SAMPLE.  A  general  principle  in  sam- 
pling is  that  the  ratio  between  the  larger  pieces  in  the  sample 
and  the  weight  of  the  sample  should  not  exceed  a  certain  amount. 


AUSTIN  S    FIRE    ASSAY.  13 

This  is  attained  by  breaking  the  pieces  finer  as  the  bulk  of  the 
sample  is  decreased. 

The  breaking  up  of  the  lumps  of  a  sample  may  be  accom- 
plished by  means  of  a  laboratory  crusher,  by  pestle  and  mortar, 
or  by  breaking  on  the  grinding-plate  with  a  hammer  or  muller. 
In  the  latter  case  it  is  well  to  provide  a  wooden  frame,  four 
inches  high,  and  but  two  inches  high  at  the  front,  which  encloses 
the  area  of  the  plate,  and  which  prevents  the  ore  from  escaping 
while  being  broken  up.  When  the  quantity  of  the  sample  is 
considerable,  some  time  and  labor  may  be  saved  by  not  breaking 
so  finely  at  first,  but  by  breaking  smaller  each  time  the  sample 
is  cut  down. 

The  work  of  cutting  down  or  of  quartering  may  be  performed 
as  follows :  The  ore  is  made  up  into  a  coned  pile  in  the  centre 
of  the  plate,  the  material  being  poured  or  thrown  upon  the  apex 
of.  the  pile  so  as  to  effect  an  even  distribution  of  fine  and  coarse. 
The  cone  is  now  flattened  out  to  a  circular  cake  or  disk  by 
scraping  the  ore  outward  every  way  from  its  apex.  Two  lines 
are  marked  at  right  angles,  dividing  the  mass  into  quarters.  The 
two  opposite  quarters  are  then  removed,  rejected,  and  the  re- 
mainder mixed  and  quartered  as  before,  taking  care,  however, 
first  to  break  the  pieces  smaller  if  necessary. 

Finally,  as  this  operation  is  continued,  there  remain  two  to  four 
ounces  of  the  ore.  This  is  ground  to  pass  an  8o-mesh  screen. 
Where  the  ore  is  rich,  spotty,  or  contains  coarse  particles  of  gold, 
it  is  better  to  use  a  I2omesh  screen.  The  sample  must  now  be 
dried,  if  riot  already  so.  The  final  mixing  of  the  sample,  after 
it  has  been  ground  up,  is  done  by  'rolling'  it  on  a  piece  of  oil-cloth 
or  of  rubber  cloth.  This  is  performed  by  laying  or  folding  over 
the  cloth  so  as  to  cause  the  ore  to  travel  from  side  to  side.  It 
is  then  rolled  at  right  angles  to  its  former  direction,  and  thus 
mixing  proceeds  until  the  operator  is  sure  that  the  ore  is  quite 
uniform  in  all  its  parts.  A  beginner  is  very  apt  not  to  move  or 
roll  the  ore  sufficiently  to  insure  its  complete  turning  over.  For 
practice,  it  would  be  well  for  him  to  mix  some  argols  and  soda 
in  this  way,  until  the  absolutely  uniform  color  of  the  mixture 
indicates  that  the  mixing  is  complete. 


14  AUSTINS    FIRE    ASSAY. 

An  expeditious  and  satisfactory  way  of  cutting  down  a  sample 
consists  in  the  use  of  the  tin  sampling-trough  or  riffle.  The  ore 
is  scattered  into  the  troughs  from  a  shovel  in  such  wise  as  not  to 
overtop  the  troughs.  Half  of  the  ore  remains  in  the  riffle,  and 
half  drops  through.  When  the  troughs  are  full  the  ore  in  them  is 
reserved,  and  more  is  again  added,  and  so  on,  until  the  sample 
has  been  all  put  through.  This  reserved  portion  again  goes 
through  the  riffle,  thus  again  halving  it.  Of  course  when  finer 
breaking  is  needed  it  must  be  done. 

Prospectors  and  others,  interested  in  mines,  often  deceive  them- 
selves by  thinking  that  by  sending  in  a  piece  or  lump  of  rock,  they 
can  arrive  at  an  idea  of  the  value  of  the  ore  they  are  going  to 
ship.  If  one  will  assay  separately  two  such  pieces  of  rock,  he 
will  see  how  much  they  vary  from  one  another.  The  only  way  to 
know  the  value  of  an  ore  is  to  sample  it  in  quantity,  so  that  all 
parts  of  the  pile  are  equally  represented. 


HI.— CARE   OF   THE  ASSAY   OFFICE. 

When  the  work  of  sweeping  out  or  cleaning  the  assay  office 
is  done  by  an  attendant,  the  assayer  should  take  notice  that  it  is 
thoroughly  done.  The  balances,  however,  to  keep  them  clean  and 
in  the  best  of  order,  need  his  personal  attention.  As  the  assayer's 
work  proceeds  through  the  day,  he  will  find  that  dust,  ashes,  and 
scattering  particles  get  on  the  desk  and  about  the  furnace.  He 
has  to  keep  his  work  going  steadily,  but  there  are  spare  moments 
when  he  can  brush  up  and  set  things  in  order  again.  In  fact, 
he  should  put  his  tools  and  appliances  in  place  again  as  he  goes 
along,  and  thus,  nothing  being  mislaid,  valuable  time  is  saved. 

It  assists  in  the  prestige  of  the  assayer  to  keep  his  office  con- 
stantly in  order.  If  this  is  done,  his  employer,  whose  good-will 
he  values,  will  notice  these  things  in  a  way  favorable  to  him; 
indeed  it  is  one  of  the  ways  of  showing  that  he  is  ambitious  and 
energetic. 

Clinkers  will  form  upon  the  interior  of  the  furnace,  and  these 
are  to  be  removed  with  a  long-handled  cutter-bar  or  chisel.  The 
assayer  also  replaces  his  own  muffles,  claying  up  the  joint  with 
fire-clay,  to  which  has  been  added  its  own  weight  of  sand,  or 
preferably,  some  coarsely  ground  fire-brick  which  he  can  break 
up  for  himself  on  the  grinding-plate.  In  a  reduction  works 
where  tailing,  slag,  or  low-grade  products  are  treated,  there 
should  be  two  cast-iron  bucking  plates,  one  of  which  should  be 
reserved  for  this  use.  In  case  a  high-grade  ore  is  followed  by  a 
low-grade  one  the  assayer  must  use  some  sand,  coke,  or  other 
similar  material  to  be  ground  on  the  plate  and  then  brushed  off, 
in  order  to  insure  the  entire  removal  of  particles  of  the  rich  ore. 
Otherwise,  low-grade  samples  may  get  'salted/  This  may  ac- 
count for  cases  where  a  trace,  or  even  more  of  metal,  has  been 
reported  when  in  fact  the  material  was  quite  barren. 

When  auriferous  placer  gravel  is  to  be  assayed,  a  considerable 
sample  of  a  known  weight,  say  100  assay  tons  (6.38  lb.),  should 


16  AUSTIN'S  FIRE  ASSAY. 

be  panned  down  carefully  and  the  residual  concentrate,  consisting 
of  black  sand,  pyrites,  and  gold,  assayed.  Even  where  the  gravel 
has  been  broken  up  and  mixed,  the  results  are  apt  to  be  uncertain, 
since  particles  of  gold  are  often  lodged  in  the  crevices  of  the 
pebbles,  and  only  water  treatment  will  wash  them  out. 


IV.— APPARATUS. 

GRINDING  PLATE  AND  MULLER.  For  breaking,  quartering 
down,  and  pulverizing  samples  of  ore  the  grinding  or  bucking 
plate  is  employed,  as  shown  in  Fig.  I.  It  is  of  cast-iron,  planed  on 
its  upper  surface,  one  inch  thick,  and  has  raised  edges.  The 
waste  part  of  the  sample  is  swept  into  a  box  placed  beneath  the 
bench.  The  muller,  which  should  weigh  25  lb.,  has  a  curved 
rubbing  surface,  and  is  rubbed  back  and  forth  on  the  plate  to 
pulverize  the  ore.  Where  the  pieces  are  one-fourth  inch  or  larger, 
they  are  broken,  using  the  muller  as  a  hammer.  Quartering  down 


FIG.  1.     GRINDING  PLATE  AND  MULLER. 

of  the  sample  is  performed  on  the  plate  using  a  piece  of  Russia 
sheet  iron  3  by  6  in.  as  a  scraper. 

Sieves  or  screens  may  be  10  in.  diam.  with  brass  screen-cloth 
and  having  a  removable  pan-bottom   (See  Fig.  2).     When  the 


i8 


AUSTIN  S    FIRE    ASSAY. 


sample  has  been  pulverized  it  is  mixed  or  rolled  on  a  piece  of 
oil-cloth  or  a  rubber  cloth  18  in.  square.  Neglect  of  this  pre- 
caution of  thorough  mixing  means  varying  results  in  duplicate 
assays.  * 


FIG.  2.     TEN-INCH  SIEVE  AND  BOTTOM. 

FIRE  TOOLS.    For  handling  crucibles,  etc.,  several  kinds  of  tools 
are  used  as  shown  below: 

Crucible  tongs  are  employed  for  setting,  removing,  and  pour- 


FIG.  3.    CRUCIBLE  TONGS. 

ing  crucibles.  They  have  bent  points  for  convenience  in  seizing 
the  crucible  and  for  pouring  them.  A  pair  of  such  tongs  is  shown 
in  Fig.  3. 


FIG.  4.     SHORT  CRUCIBLE  TONGS. 


Fig.  4  shows  the  short  tongs  used  for  removing  nails  from 
the  crucible  before  pouring.    They  are  nine  inches  long. 


AUSTINS    FIRE    ASSAY.  19 

CRUCIBLE  TONGS  FOR  MUFFLE.  (See  Fig.  5.)  These  have 
curved  ends  for  embracing  the  crucible  and  are  useful  in  putting 
in  and  removing  crucibles  (sometimes  scorifiers)  from  the  muffle. 
They  have  in  mid-length  a  flat  tongue  for  steadying  the  two  legs 
in  relation  to  one  another.  The  round  wire  tongue,  ordinarily 
provided,  is  not  satisfactory. 


FIG.  5.     CRUCIBLE  TONGS  FOR  MUFFLE  USE. 

For  handling  scorifiers  we  have  scorifier-tongs,  of  which  Fig. 
6  is  a  good  example.  It  has  a  forked  leg  as  well  as  a  flat  one, 
and  has  a  broad  spring  at  the  back.  The  flat  end  is  often  used  to 
dip  up  and  transfer  borax-glass  to  the  scorifier. 


FIG.  6.     SCORIFIER  TONGS. 


Fig.  7  represents  a  pair  of  cupel-tongs  more  carefully  made 
and  proportioned  than  those  commonly  purchased  from  dealers 
in  assay  supplies.  It  has  a  broad  and  flexible  spring  back,  and  a 


FIG.  7.     CUPEL  TONGS. 

flat  tongue  for  steadying  the  legs.  The  bent  points  should  be  no 
longer  than  here  given.  The  assayer  can  easily  cut  off  and  shape 
the  points  when,  as  is  usual,  they  are  too  long.  For  large  muffles 
they  should  be  at  least  30  in.  long. 


2O 


AUSTIN  S    FIRE    ASSAY. 


MUFFLE  SCRAPER.  This  tool,  intended  for  promptly  scraping 
out  anything  spilled  upon  the  floor  of  the  muffle,  should  hang 
near  the  muffle  so  that  it  can  be  used  without  a  moment's  delay. 
In  this  way  the  damage  to  the  muffle  can  be  greatly  lessened.  It 
is  shown  in  Fig.  8. 


FIG.  8.     SCRAPER. 

CUPEL  MOLDS.  Cupels  are  made  by  the  assayer  (though  they 
can  also  be  bought)  from  bone-ash,  ground  fine  enough  to  give 
a  smooth  surface  and  yet  have  a  porous  texture.  The  molds, 
1^2  in.  diam.,  are  in  two  parts,  the  ring  and  the  plunger,  and  are 
generally  made  of  brass.  The  assayer  must  take  care  to  handle 
them  so  as  not  to  bruise  or  deface  them  in  any  way.  It  will  be 
noticed  that  the  ring  has  an  interior  taper  for  the  easy  removal 


FIG.  9.    TWO-HOLE  SLAG  MOLD. 

of  the  cupel  when  molded,  and,  in  making  the  cupel,  the  plunger 
enters  the  smaller  diameter.     In  making  cupels,  the  bone-ash  is 


FIG.  10.    SIX-HOLE  SLAG  MOLD. 

moistened  with  a  little  water  until  it  has  a  grip  or  adhesion  such 
that,  when  molded,  it  will  cohere.  Before  use,  cupels  must  be 
carefully  dried  by  being  put  in  a  warm  place,  preferably  for  a 
month  or  more,  so  that  quite  a  stock  must  be  made  and  kept  on 
hand.  There  are  cupel-making  machines  which  will  turn  out 


AUSTINS    FIRE    ASSAY.  21 

several  hundred  cupels  in  an  hour,  but  for  a  small  assay  office, 
they  are  generally  made  in  the  simpler  way. 

SLAG  MOLDS.  They  are  made  of  various  shapes,  some  having 
a  hemispherical,  some  a  conical,  cavity.  The  most  satisfactory 
mold  is  one  having  the  conical  cavity,  smoothly  turned  out,  and  in 
a  solid  cast-iron  block,  so  as  quickly  to  abstract  the  heat  from  the 
molten  material  poured  into  it.  (See  Fig.  9.)  Fig.  10  represents 
a  similar  mold  having  six  depressions. 

Lighter  and  cheaper  molds,  having  less  iron  in  them,  are  not 
so  satisfactory. 


V.— THE  ASSAY  FURNACE. 

A  variety  of  assay  furnaces  have  been  devised,  some  using  soft 
coal  as  a  fuel  and  depending  upon  the  flame  from  the  coal  to  do 
the  heating,  some,  in  which  coke  is  used,  with  coals  in  direct 
contact  with  the  muffle  or  the  crucible,  and  some  where  the  flame 
of  gasoline  is  used,  much  on  the  same  principle  as  with  soft  coal. 
To  these  may  be  added  ordinary  illuminating  gas,  where,  as  in 
cities,  it  may  be  had. 


FlG.    11.       COAL-BURNING    ASSAY    FURNACE. 

Fig.  ii  is  an  elevation,  and  Fig.  12  and  13,  two  sectional  ele- 
vations of  a  two-high  muffle  furnace  for  burning  soft  coal,  which 


AUSTINS    FIRE    ASSAY.  23 

must  have  enough  volatile  constituents  to  produce  a  suitable 
flame.  The  interior  is  lined  with  fire-clay  tile  of  special  shapes 
so  that  the  furnace  can  be  readily  and  speedily  built.  The  rest 
of  the  furnace  is  of  red  brick,  the  whole  being  firmly  clamped 
or  bound  with  angle-iron  and  tie  rods.  The  muffles,  of  the  form 
shown  in  Fig.  12  and  13,  have  an  inside  width  of  9  in.,  and  each 


FK;.  12  AND  13.    SECTIONS  or- COAL-BURNING  ASSAY  FURNACE. 

one  is  large  enough  to  take  two  2O-gram  crucibles  (3  in.  diam. 
by  3^4  in-  high)  abreast,  and  in  at  least  four  rows,  making  twelve 
crucibles  at  a  time  or  five  rows  of  scorifiers,  four  abreast,  or 
easily  four  cupels  abreast.  The  lower  muffle,  which  can  be 
brought  to  the  higher  temperature,  is  generally  reserved  for  melt- 
ing and  the  upper  one  for  cupelling.  By  means  of  a  tile-damper 
the  assayer  can  regulate  the  heat  of  the  furnace,  so  that,  in 
cupelling,  he  can  carry  several  rows  of  cupels  at  a  time.  He  then 
subdues  the  heat  at  the  back  of  the  muffle  by  putting  in  cold 
crucibles  which  absorb  the  heat  and  cool  down  the  muffle  to 
exactly  the.  required  temperature.  The  muffles  are  carried  on  sup- 
ports, two  on  either  side.  At  the  front  will  be  noticed  a  short  flue 
for  removing  the  fumes  which  may  issue  from  the  muffles.  The 
furnace  should  have  a  stack  with  a  strong  draft,  this  stack  being 
built  independently  of  the  furnace.  The  fire-door  is  placed  so 


24  AUSTINS    FIRE    ASSAY. 

as  to  feed  the  coal  at  the  back,  and  out  of  the  way  of  the  assayer. 
It  can  also  be  arranged  so  as  to  feed  at  the  side  if  so  desired. 

With  a  slight  change  in  the  fire-box,  this  furnace  can  burn 
wood,  there  being  then  but  a  single  muffle  used.  These  furnaces 
take  16  to  24-in.  wood  as  desired. 

Where  much  work  has  to  be  done,  furnaces  having  much  larger 
muffles  are  used,  where  as  many  as  four  2O-gm.  crucibles  can  be 
put  in  abreast,  five  rows  being  melted  at  a  time. 

In  firing  with  coal  the  bed  should  be  not  more  than  6  in.  deep, 
a  little  deeper  at  the  back,  using  but  little  coal  at  a  time  and  feed- 


FIG.  14.    BROWN  PORTABLE  ASSAY  FURNACE. 

ing  frequently  so  as  to  keep  up  a  steady  flame.  Be  careful  not 
to  permit  any  holes  through  the  fuel  bed  by  which  cold  air  can 
get  up  to  cool  the  muffle.  The  fire  must  be  kept  clean  and  free 
from  ashes  and  cinders.  As  those  latter  accumulate  in  the  ash-pit 
they  must  be  removed.  Otherwise,  the  heat  from  them  softens 
and  melts  the  grate  bars,  thus  soon  destroying  them. 


AUSTIN'S  FIRE  ASSAY.  25 

Fig.  14  is  a  perspective  view  of  the  well-known  Brown  portable 
furnace  for  coke.  The  pipe  coming  from  it  is  5  in.  diam.,  and  can 
be  carried  through  the  roof  of  an  assay  office,  so  that  one  can  be 
quickly  set  up  and  put  in  operation.  It  contains  a  single  muffle. 

The  muffle  is  suited  to  scorifying  and  cupelling,  but  not  to 
receiving  crucibles.  Crucibles  are,  accordingly,  set  among  the 
coke  through  the  top  door  or  lid.  The  crucibles  are  set  in  two 
rows  parallel  to  the  muffle,  three  pointed  E  crucibles  in  a  row. 
They  are  set  to  leave  1.5  in.  between  them  and  the  side  of  the 
furnace,  and  are  packed  all  around  with  coke  to  their  tops.  It 
is  upon  the  thorough  performance  of  this  work  that  the  success 
of  the  melting  depends.  The  fire  is  lit  with  a  little  paper  and  some 
light  kindling  followed  by  heavier  pieces  at  least  an  inch  thick  and 
packed  under  and  up  to  the  muffle.  Charcoal  is  then  put  in  to 


FIG.  15.    Bos  WORTH  ASSAY  FURNACE. 

the  level  of  the  top  of  the  muffle  and  then  two  inches  in  depth 
of  coke  broken  to  egg  size  or  less.  If. left  larger,  these  pieces 
will  not  pass  down  between  the  muffle  and  the  wall  of  the  furnace. 
The  fire  is  then  lit,  and,  when  the  wood  has  burned  away,  is 
poked  down  with  a  poker  ^  in.  diam,  by  3^2  ft.  long.  More 
coke  is  then  added,  and,  when  burning,  is  worked  down  with  the 


26 


AUSTIN  S    FIRE    ASSAY. 


poker.  To  do  this  the  poker  is  thrust  down  lower  than  the  muffle 
and  is  then  carried  through  the  coke  with  a  motion  tending  to  lift 
up  the  larger  pieces  while  the  smaller  ones  fall  by  and  pack  the 
space  below  the  muffle.  Fresh  coke  is  now  put  in  up  to  the  level 
of  the  outlet  flue  and  the  crucibles  are  at  once  set,  digging  out 
a  hole  for  them  with  the  poker  and  tongs  and  setting  them  among 
the  cold  coke.  With  a  good  draft  the  coke  is  soon  burning  and 
the  charge  melting  down.  As  soon  as  the  muffle  gets  sufficiently 
hot,  cupelling  or  scorifying  can  proceed.  When  the  crucibles 
have  been  poured,  the  poker  is  again  used  to  work  down  the  coals, 
fresh  coke  is  put  in  and  a  new  batch  of  crucibles  set.  After  their 
removal,  poke  down  and  replenish  the  fire  before  cupelling. 

Even  if  there  are  no  more  crucible-melts  to  be  made,  still  the 
fire  must  be  poked  down  and  replenished  before  cupelling. 


FIG.  16.    FRONT  ELEVATION  OF  WIND-FURNACE. 

Fig.  15  represents  a  Bosworth  furnace  for  a  9  by  I5~in.  muffle. 
consisting  of  a  base,  a  body,  and  a  top,  on  top  of  which  may  be 
noticed  the  outlet  connecting  to  the  chimney  by  means  of  a  5 -in. 
stove-pipe.  The  top  is  covered  by  an  iron  door  and,  when  the 
furnace  is  in  full  operation,  it  is  filled  with  coke  to  the  sill  of  the 
feed  opening.  .  The  iron  door  covers  the  muffle,  which  latter 


AUSTINS     FIRE    ASSAY.  27 

extends  clear  to  the  back  of  the  furnace  and  is  capable  of  taking 
jthree  rows  of  2O-gm.  crucibles,  two  in  a  row,  as  seen  in  the  cut. 
The  plug  immediately  above  the  grate  is  kept  closed  except  in 
cleaning  out  the  furnace,  and  also  when  it  is  desired  to  poke  the 
glowing  coke  and  work  it ' beneath  the  muffles. 


FIG.  17.     SIDE  ELEVATION  OF  WIND-FURNACE. 

Fig.  1 6  and  17  represent  a  wind  or  natural-draft  furnace  in- 
tended for  melting  only.  A  thin  fire  or  bed  of  coke  is  first 
started  in  the  furnace.  Fresh  coke  is  added  and  the  crucibles 
are  at  once  set,  being  careful  to  place  them  to  their  full  depth  in 
the  coke,  and 'to  pack  it  around  them. 

GASOLINE  ASSAY  FURNACE.  Fig.  18  represents  in  perspective 
a  combination  gasoline  furnace  with  burner  at  the  rear,  and  the 
tank  and  pump  by  which  the  burner  is  fed.  The  furnace  proper 
has  at  the  front  a  muffle  for  cupelling,  while  at  the  top  will  be 
noticed  an  opening  with  the  cover  removed  and  showing  crucibles 
in  position  for  melting.  The  burner  (Fig.  19)  just  fits  into  an 
opening  through  the  rear  wall.  The  lower  hand-wheel  admits 
a  small  quantity  of  gasoline  to  a  tray  shown  below  the  burner, 
where  it  is  ignited  by  a  match.  On  opening  the  upper  hand- 


28 


AUSTIN  S    FIRE    ASSAY. 


wheel,  the  gasoline,  under  pressure  from  the  tank  which  has  been 
first  of  all  pumped  up,  is  heated  and  gasified  as  it  enters  the 
furnace  from  the  jet.  The  issuing  vapor  is  lit  by  a  match  imme- 


FIG.     18.     GASOLINE  ASSAY  FURNACE  WITH  TANK  AND  BURNER. 


FIG.  19.     BURNER  FOR  GASOLINE  FURNACE. 

diately  inside  the  furnace,  and  as  the  latter  heats,  this  gasification 
is  kept  up.     For  success  in  operating,  therefore,  it  is  necessary 


AUSTIN  S    FIRE    ASSAY.  29 

first  to  thoroughly  heat  the  burner.  The  tank  should  be  pumped 
to  10  or  I5~lb.  pressure.  The  crucibles  to  be  melted  may  be 
placed  within  the  furnace  while  it  is  cold  and  the  heat  brought 
up  on  them  gradually.  It  is  also  possible  to  put  them  into  a 
heated  furnace,  letting  them  take  up  the  heat,  and  finally  to  start 
the  burner.  These  furnaces  have  the  advantage  that  they  are 
quickly  brought  up  to  the  full  heat,  which  can  then  be  retained 
indefinitely  with  but  little  further  attention.  They  cost,  for  the 
furnace  $25,  for  the  burner  $11,  and  for  the  pump,  tank,  pipe, 
and  fittings,  $20. 


_,_KV 

'    OF  THE 

UNIVERSITY 

OF 


VI.— CRUCIBLES  AND   SCORIFIERS. 

For  ordinary  assaying,  but  few  out  of  the  many  kinds  adver- 
tised are  made  use  of.  Thus,  for  work  in  the  muffle,  but  two 
sizes  are  used,  the  10  and  the  2O-gm.  crucibles.  (See  Fig.  20.) 
These  crucibles  are  short  enough  to  be  placed  in  the  muffle  of  the 
coal-burning  furnaces  already  described.  Where  the  crucibles 
are  to  be  set  in  the  coke,  the  pointed  form  (Fig.  21)  is  used,  sizes 
E  and  F. 


FIG.  20.     CLAY  CRUCIBLES.     (FLAT  FORM.) 


FIG.  21.    CLAY  CRUCIBLES.     (POINTED  FORM.) 

Crucibles  are  often  used  but  a  single  time.  If  still  sound,  as 
when  assaying  silicious  ores,  they  may  be  again  used.  However, 
for  ores  or  products  containing  but  little  of  the  desired  metal, 
new  ones  should  be  employed.  Besides  the  domestic  crucibles, 
Battersea  crucibles  of  English  make  are  very  durable.  For  lead 
assays,  in  order  to  clearly  distinguish  the  crucibles  from  those 


AUSTIN  S    FIRE    ASSAY. 


31 


used  for  a  silver  assay,  the  triangular  crucible  (Fig.  22)  is  used. 
These  may  be  of  the  size  3.25  in.  diam.  by  3.5  in.  high,  called  U 
size. 


FIG.  22.     NEST  OF  TRIANGULAR  BATTERSEA  CRUCIBLES. 

Crucibles  are  made  of  fire-clay,  white  and  smooth,  or  of  a  simi- 
lar clay  into  which  has  been  mixed  a  good  deal  of  sand,  so  that 
they  are  rougher  on  the  surface  and  of  light-brown  color.  The 
latter  better  resist  the  action  of  a  silicious  charge,  the  former, 
or  clay  crucible,  the  basic  charge. 

In  scorifying,  the  2^2-in.  (outer  diameter)  scorifier,  as  shown 
in  Fig.  23,  is  employed.  They  are  good  generally  for  a  single 


FIG.  23.    SCORIFIERS. 

time  except  for  a  silicious  ore,  and,  even  at  the  first  time,  may  be 
so  corroded  as  to  permit  the  escape  of  a  charge.  The  size  speci- 
fied is  large  enough  for  assaying  one-tenth  of  an  assay  ton  of 
ore,  and,  when  larger  quantities  are  to  be  taken,  the  crucible  assay 

is  chosen. 


VIL— ASSAY  BALANCES. 

There  are  three  kinds  of  balances  used  in  assaying — the  pulp, 
button,  and  gold-balances.  In  using  any  of  these  balances  the 
variable  load  is  placed  in  the  right-hand  pan,  the  fixed  quantity 
in  the  left-hand  one.  Thus  ore  or  silver  granulations  would  be 
placed  in  the  right-hand  pan  and  buttons  or  fixed  weights  in  the 
left.  The  beam  is  raised  to  its  full  height  with  a  sudden  move- 
ment, and,  as  soon  as  the  pointer  begins  to  move,  the  assayer 
judges  what  change  is  to  be  made,  and  at  once  drops  the 
beam  while  he  makes  the  needed  change.  Only  once  does  he 
wait  for  the  double  swing,  and  that  is  when  he  is  satisfied  that 
the  weighing  is  complete.  In  this  way  great  rapidity  can  be 
obtained  in  weighing. 

Fig.  24  shows  a  pulp-balance,  sensitive  to  a  single  milligram, 
which,  with  a  2.5-in.  pan,  will  take  readily  an  assay-ton  of  ore. 
It  is  used  for  weighing  ore,  or  the  lead  buttons  of  a  lead  assay. 
Price  $15. 


FIG.  24.     PULP  BALANCE  WITHOUT  GLASS  CASE. 

Fig.  25  shows  a  pulp-balance,  enclosed  in  a  case  to  protect  it 
from  air-currents,  and  sensitive  to  0.25  mg.  It  can  therefore  be 
used,  not  only  for  weighing  pulp  and  lead  buttons,  but  even  for 


AUSTIN'S  FIRE  ASSAY.  33 

chemical  determinations,1  doing  away  with  the  necessity  of  pur- 
chasing a  chemical  balance  where  one  wishes  to  economize  in 
first  cost  of  equipment.  Price  $30. 


FIG.  25.     PULP  BALANCE  IN  A  GLASS  CASE. 

Fig.  26  represents  a  so-called  button-balance  for  weighing  the 
small  silver  buttons  obtained  in  assaying.  It  has  a  6-in.  beam, 
is  sensitive  to  1-50  mg.,  the  beam  being  graduated  to  5  mg.  for 
a  rider  of  that  weight.  In  the  drawers  may  be  kept,  on  one  side 
the  pliers  and  button-brush,  on  the  other,  sheet-lead,  copper,  and 
silver- foil  for  the  assay  of  fine  silver  bullion.  The  weights  (500 
mg.  down)  are  conveniently  kept  on  a  neatly  marked  card-board 
or  on  a  wooden  strip,  the  weights  being  returned  to  their  com- 
partments at  once  from  the  pan.  Never  get  into  the  way  of 
displacing  them.  In  handling  the  weights,  the  fingers  must  rest 
upon  the  case,  and  in  moving  the  rider,  the  fingers  must  rest 
against  the  case  to  prevent  sudden  movement.  Otherwise  the 
weights  are  speedily  bent  and  injured,  and  the  rider  may  be 
thrown  off  the  beam.  Never  leave  the  sliding  door  of  the  case 
open.  It  should  be  closed  before  you  begin  to  use  the  rider.  The 
balance  can  be  leveled  by  means  of  adjusting  screws,  watching 
the  bubble-tubes  in  the  case.  The  final  adjustment  is  made  by 


34 


AUSTINS    FIRE    ASSAY. 


means  of  a  star-wheel  or  nut  at  the  middle  of  the  beam.  It  should 
be  set  so  that  the  vibrations  are  equal  on  either  side  of  the  zero 
mark.  Carry  inside  the  balance  a  camel's-hair  brush  with  which 
to  brush  off  the  pans  before  making  the  final  adjustment.  Some- 
times, when  the  beam  seems  out  of  adjustment,  it  may  be  no  more 
than  a  speck  of  dust  in  one  of  the  pans,  or  even  on  the  beam, 
itself.  Price  $110. 


FIG.  26.    BUTTON  BALANCE. 

Fig.  27  represents  an  unusually  fine  balance  for  weighing  the 
small  particles  of  gold  which  are  left  upon  parting  the  button 
of  a  silver-gold  ore.  It  is  sensitive  to  1-200  mg.  and  has  a 
4-in.  beam  graduated  for  a  i-mg.  rider.  The  knife-edges  are  of 
agate.  For  ease  in  reading  the  rider-divisions,  pins  project  on 
the  beam  to  direct  the  eye  to  the  divisions.  A  rider  may  be 
carried  on  either  side  of  the  beam,  though,  since  it  can  travel 
half  the  length  of  the  beam,  and  read  from  the  middle  to  the 


AUSTIN  S    FIRE    ASSAY. 


35 


end,  such  an  addition  is  hardly  needed.  The  pointer  divisions 
are  viewed  through  a  reading-glass.  When  the  beam  is  dropped 
in  weighing  there  is  no  kick  or  sudden  movement  of  the  pointer, 
and  its  direction  of  movement  at  once  indicates  the  required  ad- 
justment. The  beam  is  light  and  only  4  in.  long,  consequently  the 
vibrations  are  rapid,  and  weighing  is  quickly  concluded.  Such  a 
balance  must  be  carefully  shielded  from  irregular  heating,  such 
as  the  rays  of  the  sun,  and  even  the  incandescent  globe  should 
be  suspended  centrally  above  it.  The  assayer  must  not  omit  test- 
ing to  note  whether  the  balance  is  in  perfect  adjustment  before 
weighing.  It  will  help  him  in  the  matter  of  speed  if  he  also 
knows  the  value  (in  tenths  of  a  milligram)  of  a  vibration  of  the 
pointer  on  the  divisions  of  the  scale.  Price  $200. 


FIG.  27.    FINE  GOLD  BALANCE. 

WEIGHTS.  The  weights  used  in  assaying  are  gram-weights 
and  assay-ton  weights. 

For  use  at  the  pulp-scale,  for  weighing  the  buttons  from  the 
lead  assay,  a  set  of  weights  from  50  gm.  to  10  mg.,  as  shown  in 
Fig.  28,  is  sufficient. 

For  use  at  the  button  or  gold-balances,  a  set  of  weights  from. 
500  to  i  mg.,  accurately  made,  is  sufficient. 


36  AUSTIN'S  FIRE  ASSAY. 

Assay-ton  weights,  from  one  assay-ton  to  one-twentieth  of  an 
assay-ton,  are  used  for  weighing  out  the  powdered  ore.  The 
assay-ton  equals  29.166  gram,  and  contains,  consequently,  as 
many  milligrams  as  there  are  Troy  ounces  (by  which  silver  and 


FIG.  28.     SET  OF  GRAM  WEIGHTS. 

gold  are  weighed)  in  a  ton  avoirdupois  of  2000  Ib.  Therefore, 
if  one  assay-ton  (A.  T.)  of  ore  assays  one  milligram,  the  ton 
contains  one  ounce  Troy.  The  value  is  thus  obtained :  A  pound 
avoirdupois  weighs  7000  grains  and  the  Troy  ounce  480  grains, 

therefore  we  have      ^  -  =  14.58  Troy  ounces  to  the  pound  avoir- 
400 

dupois,  or  2000  by  14.58  =  29,166  ounces  per  ton  of  2000  pounds. 
In  practice,  the  weights  of  one  gram  and  above  are  cylindrical 
and  are  handled  with  the  fingers,  those  less  than  one  gram  are 
flat,  and  are  never  touched  by  the  fingers  but  handled  with 
pincers.  Particular  care  must  be  taken  of  the  finer  weights,  and, 
in  placing  them,  the  hand  or  fingers  rest  on  the  case.  Where  a 
number  of  weights  are  to  be  removed  the  pan  is  taken  off  with 
pincers  and  they  are  dumped  from  it  upon  the  floor  of  the  case 
and  there  counted.  When  returned  to  the  box  they  are  counted 
again  as  a  check  on  the  first  reading.  In  weighing,  proceed 
systematically.  Thus  if  the  weight  is  378  mg.,  we  would  put  on 
500  mg.,  too  much,  then  200,  too  little,  200  more  too  much,  100 
too  little,  50  too  little,  20  too  little,  20  too  much,  10  too  little,  5  too 
little.  The  remainder  we  know  to  be  between  375  and  380,  which 


AUSTIN  S     FIRE    ASSAY. 


37 


is  to  be  determined  by  the  rider  which  will  weigh  five  milligrams 
on  the  beam. 

The  pincers  or  forceps  for  these  weights  are  of  brass  with  fine 
curved  points,  see  Fig.  29,  or  nickel-plated  as  in  Fig.  30.     They 


FIG.  29.     PINCERS. 


FIG.  30.     NICKEL-PLATED  PINCERS. 

are  used,  not  only  for  handling  the  weights,  but  also  for  pickmg 
up  buttons,  for  granulations,  or  other  fragments.  In  preparing 
the  silver  button  for  assay,  it  is  removed  from  the  cupel  by  means 
of  pointed-nose  button-pliers  (Fig.  31),  and  the  adherent  bone- 
ash  removed  with  the  button  brush  (Fig.  32).  For  larger 
buttons,  of  over  100  mg.,  stouter  pliers  are  to  be  preferred,  since 


FIG.  31.     BUTTON  PLIERS. 


FIG.  32.     BUTTON  BRUSH. 

the  button  must  be  gripped  so  tightly  as  to  slightly  deform  it. 
Small  buttons  may  be  removed  from  the  cupel  with  pincers,  and 
placed  on  a  smooth-faced  steel  block  (Fig.  33)  to  be  flattened 
with  a  light  hammer  (Fig.  34)  as  in  blow-pipe  work.  The  block 
is  placed  close  to  the  pan  in  the  balance  and  the  button  transferred 


AUSTINS    FIRE    ASSAY. 


from  it  directly  to  the  pan.    In  this  way  one  may  lessen  the  chance 
of  accidentally  losing  the  button. 


FIG.  33.     STEEL  ANVIL  BLOCK. 


FIG.  34.     BLOW- PIPE  HAMMER. 

For  handling  the  lead  button,  when  hammering  it  into  shape 
and  removing  the  attached  slag,  plain  steel  forceps  are  used,  about 
6  in.  long.  The  hammering  of  the  button  should  be  upon  a 
smooth-faced  steel  anvil  set  into  a  block  (Fig.  35)  weighing  4  to 
6  lb.,  and  with  a  smooth-faced  hammer  (Fig.  36)  reserved  for 
that  purpose  only,  and  weighing  at  least  one  pound. 


FIG.  35.    STEEL  ANVIL  TO  SET  IN  WOODEN  BLOCK. 


FIG.  36.    SLAGGING  HAMMER. 


SPATULA.  For  weighing  out  and  mixing  ore,  a  spatula  (Fig. 
37)  with  a  6-in.  blade  serves  very  well.  For  weighing,  an  ordi- 
nary tinned  iron  teaspoon  is  even  better,  being  cleaner  and 
quicker.  Indeed  there  are  assayers  who  use  this  also  for  meas- 
uring out  their  fluxes. 


AUSTIN  S     FIRE    ASSAY. 


39 


MOISTURE  SCALES.  For  determining  the  percentage  of  mois- 
ture in  an  ore,  or  the  weight  of  fines  and  metallics  in  a  sample, 
the  moisture  scale  (Fig.  38)  is  well  suited.  It  is  finished  with  a 


FIG.  37.    SPATULA. 

sliding  weight,  and  weighs  to  I  kg.  A  scoop  can  be  placed  on  one 
pan  and  its  counterpoise  on  the  other,  while  extra  weights  beyond 
i  kg.  can  be  added. 


FIG.  38.    MOISTURE  SCALES. 


VIII.— FLUXES   USED   IN   ASSAYING. 

At  the  temperature  of  the  assay  furnace  most  ores  are  infusible, 
and  in  order  to  make  them  fusible,  and  to  bring  them  into  a 
uniform  liquid  mass  in  which  the  precious  metals  may  come  in 
contact  with  the  collecting  agent  (molten  lead)  and  be  taken  up 
by  it,  fluxes  must  be.  added.  These  fluxes,  where  basic,  as  in 
the  case  with  soda,  will  form  a  fusible  slag  with  silica  as  a 
silicate  of  soda ;  where  acid,  they  form  a  fusible  slag  with  the 
bases,  as  when  borax  is  added.  In  addition,  some  carbona- 
ceous substance,  as  argols  or  flour,  will  reduce  litharge  to 
metallic  form. 

We  may  divide  fluxes  into : 

ACID  FLUXES,  as  borax,  glass,  or  sand. 

BASIC  FLUXES,  as  soda,  pearlash,  and  litharge. 

COLLECTING  AGENTS,  as  litharge  or  test  lead. 

REDUCING  AGENTS,  as  argols,  flour,  or  coke  dust. 

OXIDIZING  AGENTS,  as  nitre. 

COVER,  as  salt. 

BORAX.  The  ordinary  crude  borax  of  commerce  is  the  pris- 
matic neutral  variety  (Na2B4O7  -(-  10  H2O)  and  when  pure  con- 
sists of: 

Boracic   acid    (B2Oe)     36.5% 

Soda   (Na2O)    16.4% 

Water  of  crystallization    47.1  % 


100.0% 

Borax-glass  may  be  made  by  melting  this  crude  borax  in 
a  crucible,  or  in  an  iron  dish  or  ladle,  taking  care  to  fill  the  ves- 
sel only  partly,  since  the  hydrated  salt  swells  to  twice  or 
more  of  its  original  bulk.  Under  the  action  of  heat  the  water 
of  composition  is  driven  off,  leaving  white  and  opaque  crystals, 


AUSTIN  S    FIRE    ASSAY.  4! 

which  melt  down  into  a  glass.  More  borax  is  added  as  the 
former  portions  melt  down  until  the  vessel  is  two-thirds  full. 
The  molten  mass  is  now  poured  into  a  mold,  or  preferably 
upon  an  iron  plate.  The  discoloration,  due  to  iron,  where  an 
iron  dish  has  been  used,  is  a  matter  of  indifference,  the  amount 
so  dissolved  being  trifling.  When  cold,  the  borax-glass  is 
broken  in  an  iron  mortar,  and  then  ground  upon  the  bucking 
plate  until  it  will  pass  a  flour-screen  of  i6-mesh.  The  assayer 
generally  gets  it  already  prepared  for  use. 

For  crucible  assays,  crude  borax  may  be  used.  Placed  upon 
the  top  of  the  charge,  it  tends  to  keep  it  cool  to  the  last,  and  until 
the  lower  portion  of  the  charge  is  melted,  and  tends  to  prevent 
the  mechanical  loss  due  to  rapid  escape  of  the  gases.  There  is  no 
danger  to  be  feared  from  not  mixing  it  with  the  rest  of  the  charge, 
the  melting  proceeding  quite  as  well.  Where  borax-glass  is  to  be 
used  it  may  be  (and  often  is)  mixed  with  the  other  fluxes  and 
the  ore.  Borax-glass  is  most  appropriately  used  in  scorifica- 
tion,  where  it  is  placed  upon  the  top  of  the  charge  in  the 
scorifier. 

Borax  serves  as  an  acid  flux  to  unite  with  bases  contained 
in  the  ore,  and  the  quantity  to  be  used  depends  upon  the 
amount  of  bases  present,  as  may  be  estimated  either  from  the 
appearance  of  the  ore,  or  better,  by  actual  determination.  The 
common  bases  are  the  oxides  of  the  metals  as,  FeO,  ZnO, 
MnO,  CuO,  and  the  alkaline  earths  CaO,  MgO,  and  BaO.  To 
this  may  be  added  A12O3,  which  is,  however,  such  a  feeble 
base  that  it  is  quite  commonly  disregarded.  The  alkalies  and 
other  oxides,  not  above  enumerated,  occur  to  so  small  an 
extent  that  they  are  not  taken  into  account.  Alkalies  occur- 
ring in  feldspar,  would,  in  a  crucible  assay,  cut  very  little 
figure  compared  with  the  soda  added  as  a  flux.  Where  the 
amount  of  bases  can  be  determined,  it  will  be  found,  that  for 
each  part  of  base,  we  should  use  crude  borax,  four  parts,  and 
borax-glass,  two  parts.  WThen  so  used  it  makes  a  fluid  easy- 
pouring  slag,  and  takes  up  and  dissolves  the  oxidized  bases. 
Such  a  slag  is  glassy  and  brittle.  A  deficiency  of  borax  is 
indicated  by  the  slag  pouring  too  stiff  or  thick,  but  brittle. 


42  AUSTINS    FIRE    ASSAY. 

Sometimes  it  will  not  leave  the  crucible,  or  else  the  pour  is 
lumpy.  However,  a  slag  lacking  soda  may  also  act  in  this 
way,  except  that  the  silicious  slag  will  string  out  and  the 
trouble  is  to  be  laid  to  a  shortage  in  borax,  where  we  know 
that  the  amount  of  soda  is  right.  Where  a  salt  cover  is  used, 
and  crude  borax  is  employed,  the  latter  is  placed  above  the 
cover.  It  intumesces  or  swells  up,  and  here,  when  much  has 
to  be  added,  is  disposed  to  overflow  before  it  again  fuses  and 
recedes  into  the  crucible.  If  it  appears  disposed  to  thus  go 
beyond  bounds  it  may  be  pushed  back  into  the  crucible  in 
such  a  way  as  to  melt  down.  A  small  loss  of  borax  is  a  matter 
of  little  moment.  Crude  borax  loses  half  its  weight  by  fusion 
to  borax-glass,  and  hence  twice  the  weight  is  needed  to  per- 
form the  same  work  as  the  borax-glass,  as  has  already  been 
indicated. 

GLASS.  Glass  is  a  fusible  silicate  which  must  be  free  from  lead 
when  used  for  a  lead  assay.  t  It  contains  65  to  73%  silica,  and, 
being  fusible  (having  been  fused  in  the  making)  is  preferred  by 
many  to  sand  for  furnishing  silica  to  a  charge.  Glass  (besides 
silica)  contains  in  general  20%  of  soda  (Na2O)  or  of  potash 
(K2O)  together  with  lime  (CaO).  Lime,  when  present,  is  thought 
to  give  a  cleaner  separation  of  the  button,  and  hence  would  be 
an  advantage  in  assaying.  Glass  is  pulverized  to  pass  through 
a  flour-screen.  About  one-third  of  the  silica  in  glass  is  available 
as  silica,  the  rest  having  been  satisfied  by  the  alkaline  bases. 
Hence,  in  using  it,  three  grams  should  be  taken  as  equivalent  to 
one  gram  of  silica  or  sand. 

SAND.  A  clean  white  sand,  ground  and  sifted  through  a  60- 
mesh  sieve,  will  serve  to  furnish  silica  to  an  assay-charge  when 
needed.  Both  sand  and  glass  protect  the  crucible  from  being 
eaten  out  under  the  action  of  basic  fluxes  (soda  and  litharge). 
We'  may  reckon  the  whole  weight  of  a  quartz  sand  as  available 
silica,  though  it  is  a  pure  sand  which  contains  more  than  95% 
SiO2. 

Either  glass  or  sand  is  omitted  from  the  regular  assay,  since  it 
is  found  that  it  will  disturb  the  calculation  for  reducing  power, 
giving  a  button  of  a  different  size. 


43 

SODA.  While  bicarbonate  of  soda  (NaHCO3)  and  even  the 
ordinary  soda  crystals  have  been  used  in  assaying,  the  best  for 
such  use  is  the  mono-carbonate  made  by  the  ammonia  process, 
98%  pure  and  of  the  composition  Na2CO3.  When  chemically 
pure  it  contains  58.5%  caustic  soda  (Na2O).  It  forms  with  silica 
at  a  red  heat,  a  fusible  silicate  thus 

Na2CO3  +  SiO2  =  Na2SiO8  +  CO2 

the  escaping  CO2  gas  thoroughly  agitating  and  mixing  the  melt- 
ing contents  of  the  crucible.  From  the  formula  we  find  by 
calculation  that  one  part  of  silica  needs  1.76  part  of  soda  to  form 
the  mono-silicate. 

Pearlash,  or  commercial  potassium  carbonate,  contains  90% 
K2CO3  together  with  some  sodium  carbonate,  potassium  sulphate, 
and  sodic  chloride;  so  that  we  have  about  60%  of  caustic  potash 
(K2O)  available  to  combine  with  silica.  When  the  salt  is  mixed 
with  the  sodium  mono-carbonate  we  get  an  alkaline  flux  which  is 
more  fusible  than  either  of  the  carbonates  taken  separately,  and 
this  is  the  principal  advantage  which  may  be  expected  from 
such  a  mixture.  The  disadvantage  of  using  pearlash  is  the 
introduction  of  sulphates  into  the  charge,  so  that  many  assayers 
use  sodium  mono-carbonate  only.  Pearlash  is  disposed  to  absorb 
moisture  from  the  air  and  become  deliquescent,  or  at  any  rate, 
lumpy.  When  used  with  soda  it  is  mixed  in  equal  parts,  or  better, 
in  the  ratio  of  2.7  parts  of  soda  to  2.3  parts  of  pearlash.  Those 
who  use  it  claim  it  gives  more  satisfactory  results  than  soda 
only,  and  habitually  use  it  in  mixture. 

Litharge  (PbO)  is  not  only  a  basic  flux,  but  that  part  which 
is  reduced  to  lead  is  also  a  collector  of  gold  and  silver.  It  has 
the  composition  92.8%  Pb  and  7.2%  O.  Ordinary  commercial 
litharge  contains  a  small  amount  of  silver,  amounting  to  0.75  to 
1.25  oz.  per  ton.  If  an  assayer  is  compelled  to  use  this,  he  should, 
after  mixing  the  whole  lot  together  to  make  it  uniform,  determine 
the  amount  of  silver  present,  and  deduct  this  from  the  assay 
according  to  the  amount  of  litharge  used.  To  make  such  a  deter- 
mination take :  30  gm.  litharge,  2>^  gm.  argols,  and  5  gm.  sand. 
Bring  the  charge  to  quiet  fusion,  which  will  take  25  minutes. 
The  lead  button,  weighing  20  gm.,  is  cupelled. 


44  AUSTIN  S    FIRE    ASSAY. 

There  is  now  no  occasion  to  use  ordinary  litharge,  since  pure 
litharge  is  readily  obtainable  from  dealers  in  assay  supplies. 

When  unreduced,  and  entering  the  slag,  litharge  is  a  basic 
flux,  acting  upon  the  silica  thus,  PbO  +  SiO2  =  Pb  SiO3;  but 
also  exerting  a  solvent  action  upon  the  sulphides  present.  A 
portion  of  the  litharge,  in  an  ordinary  crucible  assay,  is  reduced 
for  the  purpose  of  affording  a  lead  button,  into  which  are  col- 
lected the  gold  and  silver  of  the  ore.  Before  its  reduction,  the 
litharge  has  exerted  its  oxidizing  effect,  and  the  impurities,  thus 
oxidized,  are  ready  for  solution  in  that  which  has  remained 
unreduced.  As  prescribed  in  the  regular  assay,  the  amount  of 
litharge  is  largely  increased  where  the  impurities  arsenic,  anti- 
mony, copper,  or  tellurium  are  large  in  amount.  The  reducing 
power  of  various  sulphides,  or  the  amount  of  litharge  needed 
to  oxidize  them,  is,  according  to  Berthier,  as  follows : 

Manganese   sulphide,   28  parts  lead  or  30  parts  litharge. 
Iron   sulphide    (FeS)  28     "         "      "  30     " 
Ironpyrite  (FeS2)..   46.5"         *      "  5O     " 
Copper  sulphide    . . .   23     "         "      "  25     " 

Copper   pyrite    32.5 "         "      "  35     " 

Antimony  sulphide   .   23     "         "      "  25     " 

Zinc  sulphide   23     "         "      "25     " 

Arsenic  sulphide   . . .  46.5  "         "      "  50     " 

Lead  sulphide    2     "         "      "     2     " 

Red  lead  (Pb3O4)  may  be  used  in  assaying  in  place  of  litharge. 
It  contains  90.5%  Pb,  and  has,  because  of  its  greater  percentage 
of  oxygen,  a  slightly  greater  oxidizing  power  upon  sulphides. 

REDUCING  AGENTS.  The  reducing  action  of  these  compounds, 
such  as  charcoal,  flour,  argols,  as  well  as  sugar  and  starch,  is 
due  to  their  contained  carbon  acting  thus, 

PbO  +  C  =  Pb  +  CO  (or  2  PbO  +"  C  =  2  Pb  +  CO2) 
the  carbon  monoxide  escaping  as  a  gas. 

CHARCOAL.  Well-charred  wood-charcoal  is  brought  to  a  red 
heat  in  the  muffle;  it  is  then  withdrawn,  the  ash  blown  off,  and 
the  pieces  broken  small  in  an  iron  mortar.  The  pulverizing  is 
finished  upon  the  grinding  plate  to  pass  a  4O-mesh  sieve.  One 
part  of  charcoal  will  reduce  28  parts  of  lead  from  litharge  and 


AUSTIN  S    FIRE    ASSAY.  45 

it  is  one  of  the  most  powerful  reducing  agents  known.  Its  dis- 
advantage is  its  lightness,  whereby  it  tends  to  float  on  the  surface 
of  the  charge  into  which  it  has  been  mixed. 

FLOUR.  The  flour  of  wheat  or  of  rye  may  be  used.  Its  fineness 
enables  it  to  be  mixed  intimately  with  the  charge,  and  it  is  pre- 
ferred by  many  to  argols.  One  part  reduces  about  12  parts  of 
lead  from  litharge. 

ARGOLS.  Argols  is  used  like  flour,  being  mixed  into  the  charge. 
One  part  of  it  will  reduce  about  eight  parts  of  lead  from  litharge. 
When  pure  it  has  the  formula  KHC4H4O6.  Crude  argols  con- 
tains, besides  bitartrate  of  potash,  tartrate  of  lime,  and  the 
amount  used  in  an  assay  is  so  small  that  its  fluxing  power  is  not 
regarded.  It  is,  however,  a  basic  flux.  It  has  an  advantage  that 
its  color  prevents  mistakes  in  using. 

REDUCING-FLUX  MIXTURES.  Reducing-flux  mixtures  consist 
of  a  mixture  of  any  of  the  above  reducing-fluxes  with  soda, 
pearlash,  or  the  two  together.  The  advantage  of  using  such  a 
mixture  over  that  of  the  ingredients  separately,  is  that  because 
of  its  dilution,  a  measured  quantity  will  insure  an  exact  reduction. 
Two  common  reducing-flux  mixtures  are 

80  parts  soda  to  20  parts  argols,  and 
85  parts  soda  to  15  parts  flour. 

The  mixture  should  be  intimate  and  uniform  and  the  determi- 
nation of  the  reducing  power  of  a  measureful  should  be  made. 
Pearlash  may  be  substituted  in  part  for  soda,  or  the  alkaline  car- 
bonates may  be  used  in  ratio  of  2.7  of  soda  to  2.3  of  pearlash 
(in  ratio  of  their  atomic  weights). 

TABLE  OF  REDUCING  POWERS.  One  part  of  each  of  the  follow- 
ing reducing  agents  reduces  as  indicated : 

Charcoal  -. 28     parts  lead. 

Wheat  flour    12 

Argols 8 

Reducing-flux;  80  soda,  20  argols 1.6      "         " 

Reducing-flux ;  85  soda,  15  flour 1.8      " 

This  reducing  power  varies,  and  in  any  case  it  is  best  to  deter- 
mine it  when  a  fresh  lot  has  been  prepared  as  follows : 


46  AUSTIN'S  FIRE  ASSAY. 

Mix  in  a  new  crucible  3  gm.  reducing-flux,  40  gm.  PbO,  and 
10  gm.  soda.  Melt  quickly  and  pour,  just  before  the  time  of 
quiet  fusion,  into  a  conical-hole  mold  and  weigh  the  resultant 
button.  These,  and  in  fact  any  fluxes,  should  be  passed  through 
a  flour-sieve.  Where  a  reducing  mixture  is  made,  mixing  should 
be  so  thorough  as  to  make  all  piarts  appear  quite  uniform  in 
color. 

OXIDIZING  AGENTS.  The  nitrates  of  potash  (nitre)  and  of 
soda,  especially  the  former,  are  used  for  oxidizing  the  sulphides 
of  the  charge,  where  the  ore  itself  would  reduce  too  large  a  lead 
button.  Their  oxidizing  power  is  such  that  one  part  will  reduce 
from,  litharge: 

Nitrate  of  soda  (NaNO3)  4.9  parts  of  lead. 
Nitrate  of  potash  (KNU8)  4.2  parts  of  lead. 

This  is  for  the  pure  salt.  We  reckon  the  oxidizing  power  of 
commercial  nitre  at  4  even.  Neither  of  them  contains  water  of 
composition. 

To  determine  this  oxidizing  power  prepare  two  crucibles  as 
follows :  Charge  into  each  40  gm.  PbO,  10  gm.  soda,  and  3  gm. 
argols,  and  to  one  of  them  add  3  gm.  nitre.  The  crucibles  are 
melted  quickly  and  without  loss,  poured,  and  the  resultant  buttons 
weighed.  The  difference  between  the  weights,  divided  by  3, 
will  express  the  oxidizing  power  of  the  salt.  As  a  result  of  the 
oxidizing  action  of  the  nitrates,  their  acid  portion  is  removed, 
leaving  the  alkaline  base. 

SALT.  Salt  is  used  as  a  cover  to  the  charge  in  assaying,  and 
for  this  purpose,  table  salt  or  any  ground  salt  is  well  suited.  It 
cannot  be  regarded  as  a  flux,  and  when  melted,  floats  as  a  trans- 
parent fluid  upon  the  charge.  It  then  slowly  volatilizes,  giving  a 
white  fume.  Where  the  charge  is  kept  for  a  long  time  in  the 
fire,  it  may  be  quite  driven  off.  It  will  be  found  as  a  white  layer 
upon  the  top  of  the  poured  slag.  It  acts  as  a  wash,  carrying 
down  any  particles  which  cling  to  the  sides  of  the  crucible.  The 
commercial  article  contains  some  moisture,  which  decrepitates 
on  heating.  This  may  be  removed  by  moderately  heating  the 
salt  in  an  iron  dish  before  using,  where  mechanical  loss,  due  to 
decrepitation,  is  feared. 


AUSTIN  S    FIRE    ASSAY.  47 

Many  assayers  have  given  up  the  use  of  salt,  claiming  that 
equally  good  results  are  attainable  without  it,  and  that  it  increases 
the  chance  of  loss,  especially  where  much  nitre  is  used.  Where 
salt  is  omitted,  dependence  is  placed  upon  borax  as  a  cover. 

EFFECT  OF  EXCESSIVE  FLUXING.  It  is  desirable  to  use  no  more 
fluxes  than  necessary,  since  as  the  quantity  of  slag  is  increased, 
so  is  the  silver  and  lead  therein,  and  no  slag  is  entirely  clean. 
Where  the  fluxes  are  in  excess  the  slag  is  fluid  and  thin,  and  is  not 
as  clean  as  the  normal  slag,  especially  in  the  lead  assay. 


IX.— ORES. 

In  order  to  determine  what  fluxes  to  use,  the  assayer  must  be 
acquainted  with  ores  from  their  physical  appearance. 

They  may  be  divided  into  oxidized  and  sulphide  ores. 

Oxidized  ores,  among  which  may  be  classed  the  carbonates, 
have  a  dull  earthy  look,  are  whitish  or  yellowish  in  color,  and, 
when  containing  iron,  brown  and  red.  The  depth  of  color  is 
intensified  where  manganese  is  present,  since  manganese  oxides 
tend  to  blacken  the  ore.  At  the  same  time,  ores,  apparently  oxi- 
dized, may  have  some  sulphides  mixed  with  them  and,  on  close 
inspection,  the  particles  of  zinc,  iron,  or  lead  sulphides  may  be 
detected.  A  preliminary  determination  for  reducing  power  also 
indicates  the  same  fact.  Where  there  -are  shining  particles  of 
mica,  do  not  confound  them  with  the  sulphides.  Ores  containing 
sulphur,  as  sulphate,  are  not,  however,  reducing  in  their  action. 

Sulphide  ores  have  a  semi-metallic  appearance  as  shown  in  the 
case  of  galena,  blende,  and  pyrite.  Let  us  take  the  case  of  the 
three  common  sulphides,  we  have : 

Galena   containing  87%  lead,  13%  sulphur. 

Blende    containing  67%  zinc,  33%  sulphur. 

Pyrite  containing  46%  iron,  54%  sulphur. 
It  will  be  noticed  that  the  percentage  of  metal  (or  base)  de- 
creases while  the  sulphur  increases  in  this  list.  Since  the  lead 
in  the  galena  is  reduced  to  metal  in  assaying,  it  does  not  enter  the 
slag,  and,  if  so,  from  the  point  of  view  of  the  assayer,  is  not  to 
be  regarded  as  a  base.  Galena  contains  but  little  sulphur  as  com- 
pared with  the  other  sulphides.  Of  the  remaining  sulphides 
(blende  and  pyrite),  we  may  say  that  they  contain  much  base, 
and  also  much  sulphur. 

Ores  may  be  divided  into  silicious  ores  (having  a  gritty  feel) 
and  non-silicious  ores,  or  quartz  at  one  extreme,  and  galena  or 
limestone  at  the  other.  Generally  the  non-silicious  ores  are  also 


AUSTIN'S   FIRE  ASSAY.  49 

basic,  as  lime  rock  or  iron  ore,  but  galena  or  lead  carbonate  is 
norf-silicious  but  not  basic  for  the  reason  above  given. 

Again  we  may  divide  ores  into  silicious  and  basic  ores,  since 
the  one  quality  is  just  the  opposite  of  the  other.  An  example  of 
an  oxidized  basic  ore  is  iron  ore,  and,  among  the  sulphides,  blende 
and  pyrite. 

As  regards  ores  high  in  lead  (leady  ores)  they  are  low  both  in 
silica  and  bases.  Such  ores  feel  heavy,  and,  when  a  portion  is 
weighed  out  for  assay,  it  occupies  but  little  bulk.  Thus  the 
assayer  is  warned  of  its  nature,  and,  in  case  of  a  lead  carbonate, 
he  can  judge  of  the  irony  base  it  contains  by  the  color.  When, 
however,  the  sample  has  been  dried  at  a  rather  high  temperature 
this  red  color  is  intensified,  and  this  should  be  taken  into  account. 

In  theory,  the  ore  should  be  dried  at  a  temperature  not  to 
exceed  100°  C,  since  at  higher  temperatures  sulphur  on  carbon 
dioxide  may  be  driven  off  and  the  sample  is  in  an  unstable  con- 
dition. 


X.— THE  SCORIFICATION  ASSAY. 

This  is  the  simplest  method  of  assaying,  and  is  suited  to  ail 
kinds  of  ores  which  come  to  the  assayer.  It  is  used  for  the 
determination  of  gold  and  silver  in  ores. 

In  making  a  scorification  assay  the  following  is  the  regular 
charge : 

30  to  60  gm.  test-lead,  o.i  assay-ton  of  the  ore  and  I  to  5 
gm.  borax. 

In  a  2^2 -in.  scorifier  measure  one  half  the  specified  amount  of 
test-lead,  weigh  out  the  ore,  and  mix  with  a  spatula  in  the 
scorifier.  Sprinkle  on  the  remainder  of  the  test-lead,  completely 
covering  the  mixed  portion,  then  add  the  required  amount  of 
borax.  The  quantity  of  borax  will  depend  upon  the  amount  of 
bases  in  the  ore.  When  there  is  much  base,  add  much  borax,  and 
for  little  base  use  little  borax.  For  example,  the  pyrite  or  the 
blende  ores,  already  mentioned,  need  4  gm.  borax,  while  a  silicious 
non-basic  ore  needs  but  I  gm.,  and  that  only  as  a  safeguard. 

The  scorifier,  thus  prepared,  and  marked  with  red  chalk  or 
riddle  to  distinguish  it  from  other  assays  to  be  done  at  the  same 
time,  is  placed  at  the  hottest  part  of  the  muffle,  and  the  door  of 
the  muffle  closed.  The  charge  soon  melts  down,  and,  when  in 
complete  fusion,  the  door  is  opened  and  the  scorifier  brought 
toward  the  front  of  the  muffle  to  scorify  or  'drive.'  Here  it  is 
exposed  to  the  air,  and  the  molten  lead,  at  that  high  heat,  begins 
to  oxidize,  giving  off  fumes.  As  the  operation  proceeds  litharge 
forms,  and,  together  with  the  borax,  slags  with  the  gangue  of  the 
ore.  The  slag  accumulates  upon  the  borders  of  the  scorifier,  and 
finally  there  is  left  exposed  to  the  action  of  the  air  the  middle  of 
the  bath  only.  This  centre  or  eye  diminishes  to,  say,  ^  in.  diam., 
when  this  stage  is  complete.  The  scorifier  is  again  moved  back 
to  the  hot  portion  of  the  muffle  and  the  door  closed.  As  soon  as 
the  slag  is  melted,  the  contents  of  the  scorifier  is  poured  into  a 
slag  mold.  The  scorifier,  if  the  work  is  well  performed,  should  be 


AUSTIN  S    FIRE    ASSAY.  51 

quite  free  from  any  imdecomposed  ore  and  smooth  upon  its  whole 
interior  surface. 

It  will  be  noticed  that  the  scorification  proceeds  by  three  stages : 

1.  The  ore  is  melted  down   (about  five  minutes). 

2.  The  muffle  is  opened  and  scorification  proceeds   (about 
twenty  minutes). 

3.  The  scorifier  is  again  heated   (about  five  minutes)   and 
is  ready  for  pouring. 

To  return  to  the  slag  left  cooling  in  the  mold.  When  the  con- 
tents of  the  mold  is  turned  out,  there  will  be  found  a  lead-button, 
preferably  of  10  or  12  gm.,  at  the  point  of  the  mold,  and  above 
it  the  adhering  slag.  This  slag  is  removed  by  hammering  the 
button  as  it  rests  upon  an  anvil,  so  that  the  slag  is  thoroughly 
cleaned  off  and  the  button  is  ready  for  cupelling.  It  is  to  be 
understood  that  this  button  has  gathered  up  all  the  gold  and  silver 
which  was  in  the  ore.  This  lead  is  removed  in  the  operation  of 
cupelling,  and  there  remains  behind  a  bead  or  button  of  silver 
(and  jgold). 

CHEMICAL  REACTIONS  IN  SCORIFICATION.     In  the  first  stage, 
as  the  charge  (containing  not  quite  three  grams  of  ore)   melts 
down,  air  comes  in  contact  with  it,  tending  to  oxidize  or  roast 
any  sulphides  present  according  to  the  reaction : 
PbS  +  3O      =PbO  +  SO2 
ZnS  +  30     =ZnO  +  SO2 
FeS2  +  heat  =  FeS  +  S 
The  sulphur,  on  contact  with  air,  burning  to  SO2.    Then : 

FeS  +  30  ==  FeO  +  SO2 
The  molten  lead  is  also  oxidized  to  litharge  as  follows : 

Pb  +  O  =  PbO 

But  any  PbO,  thus  forming,  eagerly  takes  up  any  silica  in  the 
gangue  of  the  ore.  The  borax  is  busy  dissolving  the  oxides  as 
they  form  thus : 

FeO   +   Na2B407   =   Na2FeB4O8 
ZnO  +  Na2BO4O7  =  Na2ZnB4O8 

By  the  time  the  door  has  been  opened,  enough  PbO  has  been 
formed  to  dissolve  the  silicious  gangue,  and  if  not,  the  increased 
access  of  air  at  this  time  speedily  produces  it.  At  the  period  of 


AUSTINS    FIRE    ASSAY. 


driving,  oxidation  of  litharge  proceeds,  which,  reacting  upon  the 
partly  decomposed  sulphides,  completes  their  roasting,  thus  : 

PbS  +  2PbO  =  3?b  +  SO2 

ZnS  +  3PbO  =  sPb  +  ZnO  +  SO, 

FeS  +  3?bO  =  3?b  +  FeO  +  SO," 

CuS  +  3?bO  =  3?b  +  CuO  +  SO~2 

As2S3  +  QPbO  =  As2O3  +  3SO2  +  pPb 

That  is  to  say,  oxidation  may  be  brought  about  by  contact  with 

the  forming  litharge  as  well  as  by  the  action  of  the  air.     As 

oxidation  proceeds,  the  slag  which  is  formed  gradually  covers 

the  surface  of  the  metal,  leaving  a  continually  smaller  eye  at  the 

centre  exposed  to  the  air.     When  the  eye  at  the  centre  becomes 

small,  oxidation  necessarily  proceeds  slowly ;  so  that,  to  save  time, 

it  is  better  to  pour  the  assay.    The  resultant  button  should  weigh 

preferably  10  to  12  gm.,  although  we  often  have  it  of  20  gm. 

weight,  and  sometimes  no  more  than  5  grams. 

GENERAL  REMARKS  ON  SCORIFICATION.  It  will  be  noticed  that 
the  quantity  of  test-lead  prescribed  for  an  ordinary  assay  is  10 
times  the  weight  of  the  ore.  When,  however,  the  ore  is  'dirty' 
(containing  arsenic,  antimony,  copper,  or  tellurium)  the  lead  is 
increased  proportionately  in  accordance  with  the  following  table : 

TABLE   SHOWING  THE  QUALITY  OF  TEST-LEAD   NEEDED  PER  ylfl  A.  T.  OF 

MINERAL. 


MINERAL 

GRAMS 
TEST  LEAD 

REMARKS 

Antimonial  ore 

45 

Arsenical  ore 

45 

Needs  high  heat  in  scorifying 

Milling-  silver  ore 

30 

Cupriferous  ore 

30  to  60 

Zinc-bearing  ore 

30  to  45 

Needs  high  heat  and  much  borax 

Leady  ore 

30 

Silicious  ore 

30 

Irony  ore 

30 

Telluride  ore 

30  to  60 

AUSTIN'S  FIRE  ASSAY.  53 

In  scorifying,  the  air,  as  it  passes  over  the  contents  of  the 
scorifier,  oxidizes  the  contained  molten  lead  to  litharge.  The  lith- 
arge reacts  on  the  impurities  of  the  ore,  oxidizing  them,  and  they 
enter  the  slag  (formed  by  the  litharge  uniting  itself  to  the  gangue 
of  the  ore  and  to  whatever  borax  has  been  added).  The  ore 
also  tends  to  float  on  the  surface  of  the  lead,  and  is  thus  rapidly 
roasted  in  contact  with  the  air.  Borax  is  added  to  the  charge 
according  to  the  quantity  of  base  present.  Much  base  needs  much 
borax.  Thus  iron  ore,  limestone,  pyrite,  and  blende  need  much 
borax  (especially  the  last,  which  tends  to  form  a  stiff  slag).  On 
the  contrary,  a  silicious  ore,  a  lead  carbonate,  or  galena,  needs 
but  little.  The  base  most  commonly  present  is  iron,  and,  in  case 
of  oxidized  ore,  the  color  gives  some  idea  of  its  quantity. 


XL—CUPELLING. 

PREPARATION  OF  THE  CUPEL.  Ground  bone-ash  is  mixed  in  a 
pan  with  a  little  water  until  there  is  a  coherence  of  the  material 
when  grasped  in  the  hand.  This  is  determined  by  experience. 
The  bone-ash  is  placed  in  the  mold,  and  the  plunger  brought 
squarely  upon  it.  A.  blow  or  two  of  the  mallet  compresses  the 
bone-ash  in  the  ring.  The  cupel  is  ejected  and  placed  upon  a 
board,  and  the  accumulated  supply  of  cupels  is  placed  in  a  warm 
place  to  dry.  They  can  be  used  after  thorough  drying. 

OPERATION  OF  CUPELLING.  The  cupel  is  warmed  upon  some 
portion  of  the  furnace,  is  freed  from  dust  by  blowing  upon  it, 
and  is  then  put  into  the  hot  part  of  the  muffle.  After  remaining 
there  until  it  has  attained  the  heat  of  the  muffle,  the  assay-piece 
is  placed  in  it  by  means  of  cupel  tongs.  These  must  grip  the 
cupel  so  as  not  to  injure  its  top  edges.  The  door  of  the  muffle 
is  closed,  and  the  lead  brought  to  its  full  heat  until  it  clears.  This 
is  due  to  the  melting  of  film  of  oxide  which  covers  the  assay- 
piece  and  which  goes  to  the  edge  of  the  cupel  leaving  the  surface 
exposed  to  the  oxidizing  influence  of  the  air.  The  door  is  opened 
and  the  cupel  brought  further  forward  toward  the  front  of  the 
muffle  to  'drive.'  When  the  muffle  is  not  so  hot  as  to  start  this 
operation  readily,  it  may  be  done  by  putting  in  a  small  piece  of 
wood  the  size  of  the  finger,  which,  laid  in  front  of  the  cupel  on 
the  floor  of  the  muffle,  burns,  producing  a  reducing  atmosphere, 
while,  at  the  same  time,  the  door  of  the  muffle  is  closed.  This 
reduces  the  lead  film  already  mentioned,  so  that  when  the  door  is 
opened,  and  the  wood  removed,  cupellation  at  once  proceeds.  The 
temperature  should  be  controlled  between  rather  narrow  limits. 
If  it  is  permitted  to  become  too  low  there  is  a  liability  of  the 
button  'freezing'  or  solidifying  and  the  assay  is  ruined.  Even  if 
the  work  can  be  pushed  to  the  hottest  part  of  the  muffle  and  the 
assay  started  again,  results  will  come  out  low,  so  that  a  button 
once  frozen,  especially  toward  the  last  of  a  cupellation,  is  useless. 
If  the  temperature  is  too  high  there  will  be  a  volatilization  loss  of 
silver,  and,  even  when  the  operation  is  well  performed,  this  loss 


AUSTIN  S    FIRE    ASSAY.  55 

will  be  as  much  as  2.5%.  The  proper  temperature  can  only  be 
learned  by  experience.  The  rule  should  be  that  the  heat  must  be 
such  that  a  slight  ring  of  litharge  crystals  (feather  litharge) 
forms  around  the  sides,  and  especially  at  the  front  interior  side  of 
the  cupel  while  the  litharge-smoke  is  seen  rising,  and  the  top  sur- 
face of  the  molten  metal  shows  a  slightly  darker  ring  around  its 
borders.  As  cupellation  proceeds,  the  lead  is  oxidized  and  at 
once  mostly  absorbed  by  the  bone-ash  of  the  cupel,  while  a  little 
passes  off  as  a  litharge-smoke.  The  button  becomes  gradually 
smaller  and,  at  the  expiration  of  perhaps  20  minutes,  begins  to 
round  up,  while,  if  the  button  is  large  enough,  a  play  of  colors 
may  be  seen  passing  over  it  as  the  last  traces  of  lead  are  removed. 
At  this  time  the  temperature  should  be  increased  by  pushing  the 
cupel  back  to  the  hotter  portion  of  the  muffle.  When  the  lead  is 
all  gone  the  play  of  colors  ceases,  the  button  brightens  or  'blicks' 
suddenly,  and  the  cupellation  is  complete.  The  cupel  may  be  left 
for  a  little  while  longer  in  the  muffle,  since  there  can  now  be  no 
further  loss  of  silver.  It  is  then  removed,  placed  on  a  mold  or 
an  iron  plate,  and  allowed  to  cool  previous  to  weighing.  If  the 
button  is  large  it  is  better  to  cover  with  a  hot  cupel  before  re- 
moving from  the  muffle  and  to  remove  it  gradually,  since  other- 
wise it  is  liable  to  'spit'  or  sprout,  which  may  result  in  a  loss 
through  the  projection  of  microscopic  particles  of  silver  from 
the  substance  of  the  button,  which  may  be  observed  under  the 
microscope  at  the  borders  of  the  cavity  of  the  cupel.  The  reaction, 
Pb  +  O  =  PbO  =  51,000  calories,  indicates  that  much  heat  is 
evolved  as  the  result  of  the  burning  of  the  lead  to  litharge. 

The  following  table  shows  the  losses  which  occur  in  cupelling 
a  button  of  10  gm.  at  different  temperatures  and  the  importance 
of  performing  this  operation  at  the  correct  temperature : 


TEMPERATURE 
DEGREES  C 

Loss 

% 

REMARKS 

700 

1.05 

Feather-litharge  about  button 

775 

1.18 

Feather-litharge  on  cooler  side 

850 

1.70 

No  more  crvstals 

925 

2.59 

«       «           « 

1000 

4.78 

«       <t           « 

XII.— PARTING. 

The  silver-gold  button  or  bead,  resulting  from  the  cupellation, 
has  now  to  be  weighed  upon  the  assay  balance  to  the  nearest 
tenth  of  a  milligram,  and  the  result  at  once  written  in  the  note 
book  in  ounces.  (The  weight  in  milligrams  is  not  to  be  written 
in.)  The  button  is  taken  from  the  cupel,  being  gripped  strongly 
in  the  pliers  (Fig.  32),  and  its  lower  surface  brushed  clean  from 
bone-ash  with  a  button-brush.  The  button  is  then  placed  on  the 
table,  the  forceps  seize  it  at  right  angles  to  the  former  position, 
and  it  is  again  brushed.  Squeezing  the  button  hard  in  the  pliers 
should  deform  it  somewhat,  and  thus  tend  to  loosen  any  particles 
of  bone-ash  sticking  to  it.*  The  button  may  now  be  flattened 
with  a  hammer,  after  which  it  is  ready  for  parting. 

PARTING  IN  A  PORCELAIN  CAPSULE.  A  Royal-Berlin  glazed 
porcelain  capsule  of  i^  in.  diam.  and  1^/4  in.  high,  is  preferably 
used  in  parting  the  button.  Upon  this  is  poured  the  pure  dilute 
nitric  acid  (i  of  acid  of  sp.  gr.  1.42  to  3  of  water),  covering  the 
button  to  say,  J4  in.  depth.  This  is  brought  to  the  boiling  point 
on  the  hot  plate,  taking  care  the  ebullition  is  not  violent,  since 
such  action  tears  apart  the  newly  released  gold.  When  action 
ceases,  manipulate  the  particles  of  gold  so  as  to  bring  them 
together.  This  can  be  effected  with  the  aid  of  a  glass  rod,  stirring 
the  liquid  in  vortex  fashion,  and  tapping  the  outside  of  the 
capsule  to  work  the  particles  together.  If  a  small  particle  of  gold 
shows  itself,  floating  on  the  surface  of  the  liquid,  it  may  be  made 
to  sink  by  touching  it  with  the  glass  rod.  It  is  always  necessary 
to  watch  that  no  such  particles  are  overlooked.  ,  The  dilute 
acid  is  poured  off  into  a  jar  or  wide-mouthed  bottle,  leaving  the 
gold  behind.  A  few  drops  of  stronger  acid  (7  of  acid  to  4  of 
water)  are  then  added  to  the  capsule,  brought  to  boiling,  until 
*Let  the  beginner  weigh  the  button  without  completely  cleaning  it, 
and  then,  after  proper  cleaning,  weigh  again.  The  difference  in  weight 
will  be  observable. 


AUSTIN  S     FIRE     ASSAY. 


57 


action  of  the  gold  has  ceased,  and  then  decanted.  The  capsule 
is  now  nearly  filled  with  hot  distilled  water  from  a  wash-bottle, 
directing  the  stream  tangentially  on  the  border  of  the  capsule 
so  as  to  disturb  the  gold  as  little  as  possible.  This  wash  water 
is  now  poured  off  and  the  operation  repeated. 

Where  distilled  water  cannot  be  obtained,  the  common  water 
will  form  a  precipitate  of  silver  chloride,  and,  when  this  happens 
with  the  first  addition  of  water,  a  few  drops  of  ammonia,  also 
added,  will  dissolve  the  chloride  and  clear  the  solution.  Where 
common  water  has  to  be  used  to  dilute  the  nitric  acid,  a  small 
button  of  silver  placed  in  the  acid  diluted  with  it  will  precipitate 
all  chlorides  present.  This  precipitate  is  filtered  off  before  using 
the  dilute  acid. 

The  capsule  is  dried  on  a  hot-plate,  and  ignited  at  a  visible-red 
heat  in  the  muffle.  It  is  then  removed,  cooled  by  setting  on  a 
cold  iron  surface,  and  is  ready  for  weighing.  In  weighing  the 
particle  of  gold,  first  see  that  the  balance  is  exactly  adjusted. 
Wipe  any  particles  of  bone-ash  from  the  exterior  of  the  capsule, 
loosen  the  gold  in  it  with  the  point  of  the  pincers,  and  tap  the 
capsule  with  the  pincers  so  as  to  cause  the  particle  to  enter  the 
pan,  which  latter  has  been  removed  and  placed  on  the  clean  floor 
of  the  balance  in  readiness  to  do  this.  Weighing  the  gold  is  done 
to  the  one-hundredth  of  a  milligram.  A  camel's-hair  pencil  brush 
may  sometimes  be  used  to  advantage  for  removing  final  traces 
of  gold.  The  results  are  registered  thus: 


1900 


Jan. 


Ontario  No.  27  (Irony  ore.) 
Ag  and  Au  15.8 

Au  3.10 

Ag 
Pb          17.2 


3.10 
12.7 


PARTING  WITH  A  FLASK.    The  button  is  dropped  into  a  part- 
ing-flask, preferably  one  of  i-oz.  capacity  (See  Fig.  39),  which 


58  AUSTIN'S  FIRE  ASSAY. 

can  stand  on  the  hot-plate.  Acid  in  moderate  quantity,  say  to 
one-quarter  inch  in  depth,  is  added,  and  the  acid  brought  to  the 
boiling  point  on  the  hot-plate.  The  action  should  be  gentle,  to 
avoid  mechanical  loss,  and  should  go  on  until  the  nitrous  acid 
fumes  are  driven  from  the  solution.  The  acid  is  poured  off, 


FIG.  39.     PARTING-FLASK. 

and  some  of  the  stronger  acid  added  in  order  to  remove  the  last 
traces  of  silver.  This  is  in  turn  brought  to  the  boiling  point  and 
then  poured  off.  Note,  that  in  all  cases  the  assayer  is  to  see  that 
the  acid  has  completed  its  work  before  pouring  it  from  the  flask. 
Turn  the  flask  so  that  the  gold  adheres  to  the  upper  side  of  it, 
and  carefully  pour  in  hot  water,  or  add  it  from  the  wash-bottle, 
to  half  fill  the  flask.  Turn  back  the  flask  to  bring  the  gold  be- 
neath the  water.  Be  careful  not  to  break  up  the  gold  by  violent 
action  of  the  water.  This  wash-water  having  been  removed,  the 
flask  is  completely  filled,  a  dry-cup  of  white  clay,  ij4  m-  diam., 
is  inverted  on  the  flask  and  the  whole  suddenly  reversed.  The 
gold  is  seen  to  fall  through  the  water  to  the  bottom  of  the  cup. 
Cant  it  a  little  to  one  side,  and  quickly  and  quietly  withdraw  the 
flask,  letting  the  water  run  out  of  it  into  a  convenient  jar.  The 
gold  in  the  cup  is  tapped  together,  if  necessary,  and  the  water 
poured  from  it.  This  work  should  be  done  promptly,  or  other- 
wise the  dry-cup  becomes  soaked  with  water,  and,  when  pouring 
off,  the  water  will  not  be  absorbed  from  the  gold.  The  cup  is 
dried,  and  ignited  at  a  low-red  heat,  and  the  gold,  now  showing 
its  characteristic  yellow  color,  is  weighed. 

Any  carelessness  in  parting  may  result  in  a  loss  of  particles 
of  gold  giving  a  low  result.     Or  again  particles  of  dust  may  get 


AUSTIN'S   FIRE  ASSAY.  59 

into  the  parting  capsule  and  if  weighed  up  with  the  gold  would 
give  a  high  result.  Where  there  are  small  amounts  of  gold,  as 
two  or  three  hundredths  of  a  milligram,  the  assayer  should  ad- 
just his  balance  just  prior  to  weighing.  He  should  never  report 
small  particles  as  a  'trace'  until,  after  weighing,  he  has  found 
that  there  is  actually  no  change  in  weight. 

Where  the  amount  of  gold  in  the  button  is  high  compared  with 
silver,  care  must  be  taken  that  the  silver  is  completely  dissolved, 
using  the  stronger  acid  after  the  weaker,  otherwise  results  will 
be  reported  too  high.  If  you  find  upon  careful  treatment  that  not 
less  than  one-fourth  of  the  original  button  is  gold,  you  may  be 
pretty  sure  that  you  have  not  dissolved  out  all  the  silver.  In 
such  a  case  it  is  necessary  to  add  to  this  button,  at  least  three 
times  its  weight  of  silver,  wrapped  in  a  small  piece  of  lead-foil, 
and  cupel  off  the  lead  in  a  small  cupel.  Or  the  same  operation 
may  be  performed  on  charcoal,  using  but  little  lead-foil.  The 
remaining  button  can  then  be  parted  properly  and  completely. 

Right  here  the  young  assayer  must  be  cautioned  not  to  commit 
the  blunder  of  reporting  as  gold  a  residue  which  may  contain 
silver.  No  one  will  pay  $20  per  ounce  for  that  which  is  worth 
but  7oc,  and  an  assayer  who  reports  in  this  way  is  liable  (and 
rightly)  to  lose  his  job. 


XIII.— THE  CRUCIBLE  ASSAY. 

While  any  ore  may  be  assayed  by  scorification,  the  amount  in 
which  the  determination  is  made  is  but  one-tenth  of  an  assay-ton, 
and  consequently,  whatever  the  error  is  in  weighing,  it  is  multi- 
plied by  ten  in  reporting  the  ounces  per  ton.  Any  variation  in 
the  sample  is  also  exaggerated  in  the  same  way.  In  the  crucible 
assay,  however,  one-half  an  assay-ton  is  commonly  used,  and 
results  become  more  accurate.  The  difficulty  may  also  be  over- 
come by  making  several  scorifications  and  uniting  the  silver 
buttons  into  one  for  the  determination  of  the  gold.  This  implies, 
however,  a  good  deal  of  extra  work,  and  the  same  result  is 
attained  by  a  single  crucible  assay. 

Assays  by  crucible  are  made  by  one  of  two  methods.  The 
first  is  called  the  'nail  assay,'  adapted  to  the  treatment  of  clean 
ores,  and  the  second,  the  'regular  assay,'  also  called  litharge 
assay,  intended  for  dirty  ores,  which  carry  impurities  to  be  got 
rid  of,  such  as  copper,  arsenic,  antimony,  or  tellurium. 

On  the  Pacific  Coast  as  much  as  two  to  four  assay-tons  of  ore 
are  fused  in  large  crucibles  in  a  coke-furnace  or  in  a  crucible- 
furnace  intended  for  melting  purposes  only.  The  method  has 
the  advantage  that  the  determination  is  made  in  a  large  portion 
of  ore,  and  consequently,  on  low-grade  ores,  with  much  accuracy. 

THE  NAIL  ASSAY  (for  clean  ores).  The  charge  is  as  follows: 
25  gm.  soda,  20  gm.  litharge,  3  or  4  gm.  argols,  5  to  15  gm. 
borax-glass,  I  to  5  nails,  and  0.5  assay-ton  of  the  ore. 

The  various  amounts  are  added  by  measure,  there  being  a 
small  cylindrical  tin  cup  provided  for  each  of  the  fluxes  of  the 
following  dimensions: 

For  soda,  i^  in.  diam.  by  1^4  in.  high. 

"     litharge,            #  "  "         "      %    "       " 

"     argols,               Y4  "  "         "      7/s   '"       " 

"     borax-glass,  I  "  "         "  i#    "       " 

"    test-lead,           #  "  "        "      #•  "      " 


AUSTIN'S  FIRE  ASSAY.  61 

These  measures  can  be  made  by  any  tinner.  They  must  all  be 
tried  for  their  capacity  when  level  full.  In  preparing  the  charge, 
the  soda,  litharge,  and  argols  are  measured  at  once  into  a  2O-gm. 
crucible.  The  ore  is  then  weighed  out  accurately  upon  the  pulp- 
scales,  and  poured  on  top  of  these  fluxes.  A  spatula  with  a  6-in. 
blade  is  now  used  to  mix  the  contents  of  the  crucible  until  they 
appear  of  a  uniform  color.  The  side  of  the  crucible  is  tapped 
with  the  handle  of  the  spatula  to  settle  its  contents,  the  borax  is 
placed  on  top  as  a  cover,  and  the  nails  are  thrust  into  the  charge, 
points  downward. 

The  crucibles,  thus  prepared,  may  be  placed  at  once  in  the  hot 
muffle,  though  it  is  well  to  place  them  in  the  muffle  when  cold  and 
allow  the  heat  to  come  up  gradually.  Where  the  crucible  is  to  be 
placed  in  the  coke,  fresh  coke  is  added  to  the  fire  before  so  doing, 
so  that  the  top  part  of  the  crucible  is  cold.  This  diminishes  the 
chance  of  boiling  over  and  consequent  loss.  The  crucible  remains 
in  the  fire  until  its  contents  are  in  tranquil  fusion,  the  time  vary- 
ing, in  general,  from  twenty  minutes  to  half  an  hour.  Sometimes, 
however,  the  time  is  greatly  exceeded,  as  when  an  occasional 
bubble  is  seen  rising  through  the  molten  charge.  In  any  case 
the  charge  should  be  hot,  well  fused,  and  liquid,  and  should  pour 
smoothly,  without  lumps  and  yet  not  be  too  thin  or  easily  flowing. 
After  pouring,  the  assayer  should  look  into  the  crucible  to  see 
that  it  is  perfectly  smooth  and  with  no  unfused  matter  sticking 
to  its  sides.  Sometimes  scale  from  the  nails,  or  a  piece  of  one  of 
them,  may  adhere  to  the  crucible,  but  this  can  be  scraped  out  with 
the  cupel  tongs.  Upon  removal  of  the  crucible  from  the  furnace, 
hold  it  by  the  tongs  in  the  left  hand,  and,  with  the  right,  use  a  pair 
of  short  tongs  or  forceps  (Fig.  4)  to  take  out  the  nails.  Now, 
still  holding  the  crucible,  tap  it  upon  the  table  or  plate  where  you 
are  doing  the  pouring,  and  then  neatly  pour  into  a  conical  mold 
(Fig.  10).  After  pouring,  do  not  disturb  the  mold  for  five 
minutes  or  more,  or  until  the  contents  have  cooled  and  set.  If 
you  have  to  move  the  mold,  keep  it  perfectly  level.  Otherwise 
you  have  the  lead  forming  a  fin  at  the  side  of  the  mold.  When 
set,  the  cone  of  slag  and  the  button  are  slid  out  on  the  grinding 
plate,  or  on  an  anvil,  the  button  is  detached  from  the  slag  and 


62  AUSTIN'S   FIRE  ASSAY. 

hammered  into  the  form  of  a  cube.  The  object  of  the  hammering 
is  to  remove  the  slag,  and,  when  the  button  is  thus  cleaned,  it  is 
ready  for  the  next  operation  of  cupelling.  Any  slag  left  adherent 
to  the  button  shows  itself  when  cupelling  as  a  slight  crust  at  the 
edges  of  the  cupel,  and  many  retain  a  little  lead,  thus  causing  a 
loss. 

Cupelling  should  be  done  at  a  low  temperature  so  as  to  make 
feather-litharge  upon  the  surface  of  the  cupel,  otherwise  there 
is  an  increased  cupellation-loss  by  volatilization.  For  gold,  this  is 
less  important.  Toward  the  end  of  the  cupelling,  and  as  the  silver 
button  begins  to  show  up,  the  cupel  should  be  shoved  back  an  inch 
or  two  to  where  the  muffle  is  hotter.  This  is  to  insure  that  the 
last  traces  of  lead  are  removed.  By  close  watching  you  may  see 
the  'blick'  or  sudden  brightening  of  the  silver  bead  which  remains 
upon  the  cupel.  Some  assayers  think  that  at  this  stage  it  is 
essential  to  remove  the  cupel  from  the  furnace,  but,  evidently, 
when  the  silver  button  has  become  solid,  as  it  now  does,  no  further 
silver  loss  can  occur. 

The  silver  bead,  when  brushed  to  free  it  from  bone-ash,  may  be 
weighed,  then  flattened,  dropped  into  hot  dilute  nitric  acid  in  the 
parting  capsule,  and  parted,  as  already  described  under  the  head 
of  scorification.  If,  however,  the  amount  of  gold  by  weight 
approximates  one-fourth  of  the  button,  this  treatment,  with 
weaker  acid,  must  be  followed  by  one  with  acid  of  1.3  sp.  gr. 
(7  parts  strong  acid  to  4  parts  water).  When  the  gold  is  in 
excess  of  this  specified  amount,  parting  proceeds  slowly  and 
imperfectly,  and  the  silver  cannot  all  be  removed.  In  such  a 
case  the  button  is  wrapped  up  in  two  or  three  grams  of  lead 
foil  together  with  a  little  silver  foil,  in  quantity  such  that  the 
total  silver  present  shall  be  at  least  three  times  that  of  the  con- 
tained gold.  This  is  quickly  cupelled  on  the  old  cupel  or  on 
another  small  one  of  one  inch  diameter.  The  resultant  button 
can  now  be  easily  parted.  Where  it  is  not  desired  to  determine 
the  silver,  but  the  gold  only,  a  small  piece  of  silver  foil  may  be 
added  to  the  lead  button  just  before  cupelling,  or  it  may  be  added 
in  the  crucible. 

ACTION  OF  THE  FLUXES.  As  regards  the  action  of  the  soda  and 
borax,  an  easy  aid  to  the  memory  is  to  say:  S  equals  S  and  b 


AUSTIN'S   FIRE   ASSAY.  63 

equals  b,  or  that  the  soda  is  intended  to  flux  the  silica  and  the 
borax  the  bases.  In  an  extremely  silicious  ore  the  slag,  even  with 
a  good  heat,  may  be  stiff,  so  that  when  poured  it  runs  with  dif- 
ficulty (sometimes  not  at  all)  from  the  crucible.  Now,  the  25  gm. 
soda  already  prescribed  is  sufficient  for  all  ores  except  the  very 
silicious  ones,  and  when,  as  in  the  above  case,  this  trouble  threat- 
ens, it  may  be  got  over  by  adding  another  10  gm.  soda  direct  to 
the  crucible  while  still  in  the  furnace.  The  addition  of  borax 
will  not  help.  When  grinding  up  the  ore  on  the  plate,  its  gritty 
feel  is  a  sign  that  the  ore  is  silicious.  An  ore  consisting  of  lime- 
stone, dolomite,  or  heavy  spar,  while  of  a  light  color,  would  not 
have  a  gritty  feel  under  the  muller. 

On  the  other  hand  an  ore  which  the  assayer  knows  to  be  basic, 
if  acting  in  this  way,  would  need  more  borax  (but  not  more  soda) 
added  to  the  charge,  in  order  to  improve  its  fluidity. 

The  way  the  scarcely  fused  slag  acts  is  significant.  When  it 
can  be  strung  out  in  threads  like  molten  glass,  it  is  silicious,  but 
when  it  breaks  short,  as  though  brittle,  in  a  way  to  be  learned 
by  observation,  it  is  too  basic. 

Referring  to  the  table  and  the  comments  thereon  (Chapter  IX), 
it  will  be  noticed,  that  in  the  nail-assay,  the  lead  is  not  to  be 
regarded  as  a  base  which  will  go  into  the  slag,  and  hence  no 
borax  is  to  be  provided  for  because  of  its  presence.  With  zinc 
and  iron  present,  as  in  blende  or  pyrite,  in  calamine  or  in  iron 
ore,  or  with  the  alkaline  earths  as  in  heavy  spar  and  limestone, 
it  is  a  different  matter,  since  these  bases  enter  the  slag  and  they 
need  borax  to  flux  them.  The  amount  of  borax  prescribed  is  from 
5  to  15  gm.  for  the  half  assay-ton  of  ore  taken.  The  smaller 
amount  of  5  gm.  is  no  more  than  enough  to  form  a  cover  to  the 
charge,  though  enough  for  small  amounts  of  bases.  The  maxi- 
mum amount  of  15  gm.  is  to  be  used  where  there  is  much  base 
present.  In  particular,  when  there  is  much  blende,  we  would  use 
the  maximum  amount  of  borax,  since  blende  is  disposed  to  make 
a  stiff  slag. 

In  the  nail-assay,  enough  litharge  is  used  to  form  a  lead  button 
of  1 8  to  20  grams,  which  is  large  enough  to  remove  or  collect  all 
the  silver  or  gold  which  the  ore  contains.  Since  an  excess  of 


64  AUSTIN'S  FIRE  ASSAY. 

argols  is  also  used,  this  lead  is  all  reduced.  Should  there  be  lead 
in  the  ore,  this  would  also  go  into  the  lead  button,  and  hence  the 
assayer  must  take  care  correspondingly  to  lessen  the  litharge. 
For  example,  in  the  case  of  a  galena  carrying  say  70%  lead 
there  would  be  10.5  gm.  lead,  and  hence  only  half  the  usual 
quantity  of  litharge  would  be  needed.  There  is  considerable 
lee- way  allowable  in  the  size  of  the  button.  As  little  as  15  or  as 
much  as  25  gm.  will  equally  serve,  but  when  below  the  former 
amount,  it  is  doubtful  whether  all  the  gold  and  silver  is  collected, 
while,  where  the  button  is  so  large  as  25  gm.,  it  is  longer  in 
cupelling,  and  volatilization  loss  increases. 

Since  the  reducing  power  of  a  gram  of  argols  is  such  as  will 
reduce  approximately  8  gm.  lead,  the  amount  of  3  or  4  gm.  pre- 
scribed will  be  in  excess  of  all  requirements,  so  that  all  the  lead 
present  in  the  charge  is  bound  to  be  reduced. 

The  limits  in  the  number  of  lo-penny  nails  are  from  one  to  six, 
and  the  number  is  dependent  upon  the  quantity  of  sulphur  present. 
Where  the  ore  consists  of  pyrite,  or  of  blende,  the  larger  number 
of  nails  is  used,  in  the  case  of  galena  two,  while  those  ores  con- 
taining little  or  no  sulphur  need  but  a  single  nail.  One  nail  at 
least  is  used,  since  it  may  be  possible,  even  in  a  supposedly  oxi- 
dized ore,  that  there  is  still  a  little  sulphur.  At  any  rate  a  nail  can  do 
no  harm.  It  cannot  be  said  that  too  many  nails  are  an  objection, 
except  as  they  take  time  to  remove  from  the  crucible  before 
pouring,  and  that  a  drop  of  lead  from  the  button  might  become 
adherent  to  one  of  them. 

To  show  how  much  borax  and  how  many  nails  are  needed  in 
assaying  sulphide  ores,  let  us  take  the  case  of  the  following 
sulphides.  They  contain,  when  pure,  as  follows : 

Galena         PbS .87%  Pb 13%  S 

Blende         ZnS 67%  Zn 33%  S 

Pyrite          FeS 47%  Fe 53%  S 

It  will  be  seen,  that  starting  with  blende,  its  sulphur  is  33%. 
'The  zinc  makes  up  the  remaining  67%  and  is  twice  the  sulphur, 
that  is,  in  blende,  the  sulphur  makes  up  one-third,  the  zinc  two- 
thirds.  Next  we  find  the  quantities  of  base  varying  by  20%  up 


AUSTIN'S  FIRE  ASSAY.  65 

and  down.  Thus  all  the  numbers  are  deduced  if  we  know  the 
sulphur  in  blende. 

Now  as  to  the  action  of  borax.  Referring  to  our  data  we  need 
not  consider  the  lead  as  a  base,  since  it  enters  the  button,  not 
showing  up  in  the  slag  at  all.  Of  the  two  remaining  bases  the 
zinc  is  more  basic,  blende  needing  more  borax  than  pyrite.  For 
the  sulphur  contents  we  would  conclude,  that  the  pyrite  and 
blende  need  the  maximum  amount  of  six  nails,  the  galena  two. 
As  a  matter  of  fact  we  use  many  for  both  blende  and  pyrite,  and 
few  for  galena. 

CHEMICAL  REACTIONS  DURING  FUSION.  Lead  oxide  is  acted 
on  by  the  carbon  in  the  argols,  the  reaction  being  as  follows : 

PbO  +  C  =  Pb  +  CO 
CO  +  PbO  ==  CO2  +  Pb 

The  button,  thus  reduced,  carries  down  with  it  the  gold  and 
silver  contained  in  the  ore.  If  copper  and  antimony  are  present, 
they  are  reduced  also,  making  a  hard  button,  if  there  is  much  of 
either  metal.  When  such  a  lead  button  is  cupelled,  some  gold 
or  silver  is  carried  into  the  cupel,  so  that  the  nail-assay  is  suited 
only  to  clean  ores. 

Silica,  itself  infusible  at  high  temperatures,  is  fluxed  by  soda 
thus: 

SiO2  +  Na2CO3  =  Na2SiO3  +  CO, 

the  carbon  dioxide  passing  off  as  a  gas.  By  calculation,  there 
must  be  26  gm.  sodium  carbonate  to  form  the  mono-silicate  where 
there  is  present  15  gm.  (nearly  one-half  assay-ton)  silica  as  in  the 
case  of  a  pure  quartz  ore. 

If  crude  borax  is  used  in  the  assay,  its  water  of  composition  is 
first  given  off  thus: 

Na2B4O7   +   ioH2O   +   heat   =   Na2B4O- 
At  first  it  swells  up,  sometimes  to  the  extent  of  overrunning 
the  side  of  the  crucible,  then  melts  down  into  borax-glass,  hence 
the  advantage  of  using  this  latter.     Borax  has  a  solvent  e.ffect 
upon  bases,  much  as  follows: 

FeO  +  Na2B4O7  =  Na2FeB4O8 
CaO  +  Na2B4O7  =  Na2CaB4O8 


66  AUSTIN'S  FIRE  ASSAY. 

forming  fusible  berates.  Borax,  however,  dissolves  bases  in  all 
proportions. 

The  nails  act  upon  the  sulphides  much  as  follows : 

FeS2  +  Fe  =  2FeS 

The  iron  sulphide  or  matte  thus  formed  is  dissolved  by  an 
alkaline  carbonate,  hence  the  necessity  of  having  plenty  of  soda 
at  all  times  in  the  charge.  Galena  is  decomposed  as  follows : 

PbS  +  Fe  =  Pb  +  FeS 
and  for  lead  in  oxidized  form 

PbO  +  Fe  =  FeO  +  Pb 

so  that,  even  were  there  no  argols  present,  the  lead  would  be 
reduced  to  metallic  form.  Blende  is  decomposed  according  to 
the  equation : 

ZnS  +  Fe  -f  PbO  =  FeS  +  ZnO  +  Pb 

THE  REGULAR  OR  LITHARGE  ASSAY  (also  called  'nitre  assay'). 
This  is  used  for  impure  ores,  or  those  containing  certain  im- 
purities as :  -As,  Sb,  Fe,  and  Cu,  which  must  be  eliminated  from 
the  lead  button  before  cupelling.  To  do  this  the  following  charge 
is  made  up: 

25  gm.  soda  (mono-carbonate),  30  to  120  gm.  litharge,  0.5 
A.  T.  ore,  3  to  15  gm.  borax-glass,  with  argols  or  nitre  to  suit. 

The  quantity,  either  of  argols  or  nitre,  which  is  to  be  used  will 
depend  upon  the  reducing  power  of  the  ore,  and  is  so  regulated 
that  a  lead  button  of  20  gm.  is  reduced  from  the  litharge  in  the 
charge. 

To  ascertain  this  reducing  power  we  take  a  preliminary  charge 
as  follows: 

o.i  A.  T.  ore,  40  gm.  litharge,  and  10  gm.  soda. 

This  is  melted  down  rapidly  in  a  lo-gm.  crucible,  withdrawing 
it,  even  before  quiet  fusion,  and  pouring  into  a  conical  mold. 
The  lead  button  obtained  will  vary,  from  nothing  if  it  is  an 
oxidized  ore,  to  upward  of  30  gm.  if  the  ore  is  an  iron  pyrite, 
the  reducing  action  being  due  to  the  presence  of  sulphur  in 
sulphides.  If  the  reducing  power  of  this  one-tenth  be  multiplied 
by  five  we  have  that  of  a  half  assay-ton.  We  may  have  either: 


AUSTIN'S   FIRE  ASSAY.  67 

1.  The  reducing  power  of  one-tenth  A.  T.  less  than  4.     In 
this  case  multiply  by  5,  subtract  the  product  from  20,  and  divide 
the  remainder  by  the  reducing  power  of  the  argols,  say  8.    This 
will  give  us  the  number  of  grams  of  argols  to  be  used.     For 
example,  let  the  reducing  power  be  2.     Then 

20—  (2  X  5)  =  I-2S  gm-> 

8 
the  quantity  of  argols  to  add  in  the  regular  assay. 

2.  In  the  second  case,  the  reducing  power  being  greater  than 
4,  we  multiply  by  5,  subtract  20  from  it,  and  divide  the  remainder 
by  4,  the  oxidizing  power  of  nitre.     Thus  suppose  the  reducing 
power  to  be  22 ;  then  we  have 

(22  X  5)  —  20  —  22.5  gm. 

4 
to  be  added  to  the  charge  in  order  to  secure  a  button  of  20  grams. 

Returning  to  a  consideration  of  the  regular  charge  we  may 
note: 

The  soda  is  added,  as  in  the  nail-assay,  to  flux  the  silica. 

The  litharge  varies  according  to  the  impurities  in  the  ore, 
which  may  be  estimated  from  the  appearance  of  the  ore,  or  from 
experience  with  it.  For  an  ore  containing  but  little  of  them,  we 
may  use  30  gm.,  while  for  ores  carrying  much  arsenic,  antimony, 
tellurium,  or  copper,  we  use  to  the  limit  of  120  gm.  PbO.  It  will 
be  seen,  that  since  but  20  gm.  is  reduced  from  the  litharge,  there 
will  be  left  anywhere  from  10  to  100  gm.,  which  goes  into  the 
slag.  It  is  this  litharge  which  acts  on  these  impurities,  oxidizing 
and  dissolving  them. 

The  borax-glass  acts  precisely  as  in  the  nail-assay,  forming 
double  berates  with  the  bases. 

This  charge  is  placed  in  a  2O-gm.  crucible,  is  mixed  there  with 
a  spatula,  and  the  borax  sprinkled  upon  it  to  form  a  cover.  It 
is  placed,  either  in  the  muffle  or  among  the  coals  of  a  wind- 
furnace,  brought  to  fusion,  but  at  such  a  rate  as  not  to  permit 
any  of  its  contents  to  escape  from  the  crucible  or  to  boil  over, 
and  finally  brought  to  quiet  fusion.  This  work  will  take  from 
one-half  to  one  hour.  There  is  no  objection  to  retaining  the  melt 


68  AUSTIN'S  FIRE  ASSAY. 

in  the  furnace  even  after  this  time.  On  the  other  hand,  if  the 
crucible  is  withdrawn  before  reduction  is  complete,  not  all  the 
silver  is  obtained.  In  pouring,  rest  the  lip  of  the  crucible  upon 
the  mold,  and  pour  at  an  even  rate.  If  the  slag  appears  to  be 
stiff,  when  melting  down  an  ore  known  to  be  silicious,  it  may 
be  made  more  fluid  by  the  addition  of  soda.  For  a  known  basic 
ore,  on  the  contrary,  we  must  add  borax  to  produce  the  same 
effect.  This  addition  may  be  made  by  putting  in  the  amount  by 
means  of  a  long-handled  spoon  forged  on  the  end  of  an  iron  rod 
two  feet  long,  or,  in  an  off-hand  way,  by  means  of  a  scorifying 
dish  held  by  the  crucible  tongs.  The  button  which  results  from 
this  assay  should  be  soft,  and  when  cupelled,  show  freedom  from 
impurities.  If  not  in  this  condition,  the  assay  should  be  repeated, 
using  a  much  larger  quantity  of  litharge. 

CHEMICAL  REACTIONS  OF  THE  REGULAR  ASSAY.  A  sulphide 
ore,  having  a  high  reducing  power,  is  permitted,  in  the  regular 
assay,  to  act  upon  and  reduce  from  litharge  20  gm.  lead,  much  as 
follows : 

FeS2  +  5?bO  =±  5?b  +  FeO  +  2SO2 
PbS    +  2PbO  =  3?b  +  SO2 
ZnS    +  3?bO  =  3?b  +  SO2  +  ZnO 

The  sulphur  dioxide  escapes  as  a  gas,  and  the  oxides  are  dis- 
solved by  the  borax.  But  there  remains  a  further  amount  of  the 
sulphides,  for  the  oxidation  of  which  nitre  has  been  provided, 
and  which,  at  a  high  temperature,  is  decomposed. 

2KNO3  =  K2O  +  2NO  +  30 

the  oxygen  thus  furnished  acting  directly  upon  the  sulphides  as 
for  example : 

FeS2  +  5O  =  FeO  +  2SO2 
CuS  +  30  =  CuO  +  SO2 
Also 

SO2  +  O  =  SO3 
K20  +  S03  =  K2S04 
and  this  floats  on  top  of  the  charge. 

We  may  note  here,  that  where,  in  the  nail-assay,  copper  is  re- 
duced to  metallic  form  entering  the  button,  it  is  here  oxidized 


AUSTIN'S  FIRE  ASSAY.  69 

and  goes  into  the  slag.  Arsenic  and  antimony  sulphides  are  also 
oxidized  according  to  the  formula : 

As2S3  +  90  =  As203  +  3S02 

Sb2S3  +  90  =  Sb203  +  3S02 
and  enter  the  slag,  being  dissolved  by  the  litharge. 

Oxidized  ores  have  no  reducing  power,  and  to  them  we  add 
argols,  which  act  as  in  the  nail-assay: 

PbO  +  C  =  Pb  +  CO 

We  are  careful  in  this  case  to  add  no  more  (as  may  be  determined 
by  a  preliminary  assay)  than  will  reduce  the  2O-gm.  button,  since 
it  is  essential  that  there  be  litharge  to  enter  the  slag  to  dissolve 
impurities. 


XIV.— ROASTING  OF   ORES. 

Where  an  ore  contains  but  little  gold,  and  we  wish,  therefore, 
to  determine  it  in  a  large  quantity,  the  ore  may  be  first  roasted. 
To  do  this,  weigh  out  one  assay-ton  of  the  ore  and  put  it  in  a 
4-in.  roasting-dish.  The  dish  is  first  carried  at  a  low  heat  at  the 
front  of  the  muffle.  Where  there  is  much  pyrite  present  it  will 
snap  and  fly,  causing  mechanical  loss.  This  may  be  guarded 
against  by  partly  covering  the  dish  with  a  crucible  cover,  leaving 
the  front  side  of  the  dish  exposed.  The  dish  is  gradually  ad- 
vanced into  the  muffle,  finishing  the  ore  at  a  red  heat,  but  so  that 
it  is  not  fused  together.  During  the  roasting  the  contents  of  the 
dish  are  stirred  several  times  to  expose  fresh  surfaces  to  the 
action  of  the  air.  Roasting  may  be  regarded  as  complete  when 
no  odor  of  sulphur  is  observable  upon  removing  the  dish  and 
smelling  of  its  contents. 

CHEMISTRY  OF  ROASTING.  The  ore  may  consist  of  a  silicious 
gangue  together  with  sulphides  of  iron,  copper,  zinc,  and  lead. 
Of  these,  pyrite  is  the  most  easily  decomposed,  the  first  equivalent 
of  sulphur  being  driven  off  at  a  low  temperature  thus : 

FeS2  +  heat  =  FeS  +  S 

The  sulphur,  as  it  is  evolved,  encounters  the  air  and  is  burned 
to  SO2.  With  an  abundance  of  air,  and  at  an  increased  tem- 
perature, the  iron  sulphide  is  roasted  as  follows : 

3FeS  +  iiO  =  2SO2  +  Fe2O3  +  FeSO4 

The  odor  from  the  escaping  sulphuric  dioxide  is  quite  evident. 
We  also  have  when  there  is  chalcopyrite, 

FeCuS2  +  6O  =  CuO  +  FeO  +  2SO2 

As  the  heat  is  increased  to  590°,  the  iron  sulphate  becomes 
decomposed,  transferring  its  SO3  to  the  copper  thus, 

FeSO4  +  CuO  =  FeO  +  CuSO4 


AUSTIN'S   FIRE  ASSAY.  71 

The  ferrous  oxide  is  then  further  oxidized,  giving,  where  much 
of  it  is  present,  a  reddish  color  to  the  roast,  namely, 

2FeO   +  O  =   Fe2O3 
Zinc  and  lead  sulphides  are  roasted  as  follows : 

ZnS  +  30  =  ZnO  +  SO2 
PbS  +  30  ==  PbO  +  S02 

and  at  a  still  higher  temperature  (750°)  the  copper  sulphate  is 
decomposed,  transferring  its  sulphuric  anyhydride  to  the  zinc 
and  lead  compounds,  and  forming  sulphates, 

CuSO4  +  ZnO  =  ZnSO4  +  CuO 
CuSO4  +  PbO  ==  PbSO4  +  CuO 

These  newly  formed  sulphates  are  hard  to  decompose  (above 
1000°  being  needed)  by  the  action  of  heat,  especially  since  the 
finishing  heat  must  be  high,  silver  will  be  volatilized.  When  so 
roasted,  these  sulphates  do  not  affect  the  reducing  power  of  the 
ore.  Antimony  and  especially  arsenic  are  oxidized,  forming 
volatile  compounds,  and  are  thus,  in  part  at  least,  removed. 
As2S3  +  90  =  As2O3  -|-  3SO2 

Ores,  because '-of  the  time  and  attention  needed,  are  seldom 
roasted;  the  assayer  generally  takes  the  shorter  method  of  a 
nail  or  regular  assay,  making  them  in  duplicate,  and  combining 
the  cupelled  buttons  when  the  amount  of  gold  is  but  small. 

THE  ASSAY  OF  ROASTED  ORE  FOR  SILVER  AND  GOLD.  One  assay- 
ton  of  ore  has  been  roasted  so  that  it  is  oxidized  and  has  lost 
its  reducing  power.  We  accordingly  take :  I  A.  T.  ore,  40  gm. 
soda,  40  gm.  litharge,  20  gm.  borax,  and  2.5  gm.  argols. 

This  is  placed  in  a  No.  F  crucible,  if  it  is  to  be  done  in  a  wind- 
furnace,  the  borax  being  used  as  usual  as  a  cover.  The  excess 
of  litharge,  over  that  needed  to  form  the  lead  button,  is  intended 
to  take  care  of  the  impurities  present.  Where  there  is  much  of 
these,  the  roasting  method  would  not  be  followed. 

The  charge  is  melted  down  to  quiet  fusion,  and  is  then  poured 
into  a  conical  mold.  The  quantity  of  slag  may  be  so  much  as  to 
overflow  the  mold,  but  this  does  not  matter.  A  2O-gm.  button 
should  result.  In  other  respects  the  assay  is  conducted  as  in  the 
regular  assay. 


XV.— ASSAY  OF  MATTE. 

This  is  an  artificial  iron-copper  sulphide,  a  furnace  product 
consisting  of  iron  and  sulphur,  the  iron  being  more  or  less  re- 
placed by  copper  and  perhaps  lead.  It  contains  silver  and  gold 
taken  from  the  ore-charge,  and  is  assayed  by  scorification  using 
45  to  60  gm.  test-lead,  o.i  A.  T.  the  ore,  and  2  to  3  gm.  borax. 

The  higher  the  matte  in  copper,  the  more  test-lead  we  add.  In 
cupelling  one  can  tell  by  the  appearance  of  the  cupel  whether  the 
copper  has  been  entirely  removed  from  the  button,  since  at  the 
finish  the  surface  of  the  cupel  immediately  around  the  button  is 
a  little  lighter  in  color  than  the  rest.  Also,  upon  dissolving  the 
button,  if  the  solution  is  neutralized  with  a  little  ammonia,  the 
absence  of  blue  color  denotes  freedom  from  copper. 


XVI.— HIGH-GRADE   SILVER-SULPHIDE  ASSAY. 

The  assayer  may  be  called  upon  to  assay  ores  rich  in  silver, 
or  silver  sulphides  of  several  thousand  ounces  value,  the  product 
of  a  silver-lixiviation  process.  The  sulphides  may  contain  much 
copper,  which  has  been  precipitated  at  the  same  time  with  the 
silver,  so  that  we  scorify  the  sulphide,  making  a  charge  as  fol- 
lows:  o.i  A.  T.  ore,  45  gm.  test-lead,  and  2  gm.  borax-glass. 

It  will  be  noticed  that  the  usual  quantity  of  30  gm.  test-lead 
has  been  increased  because  of  the  presence  of  copper.  A  clean 
ore,  no  matter  how  rich,  can  be  quite  as  well  assayed  with  the 
usual  30  gm.  The  borax-glass  is  added  to  take  care  of  the  copper 
oxide. 

.When  the  button  resulting  from  thfs  scorification  has  been 
cupelled,  the  litharge-soaked  part  of  the  cupel  is  saved,  together 
with  the  slag  of  the  scorification.  These  two  are  then  coarsely 
powdered  in  a  mortar,  or  on  the  grinding  plate,  to  about  2O-mesh 
and  assayed  with  the  following  charge :  Slag  and  cupel,  30  gm. 
litharge,  20  gm.  soda,  2.5  gm.  argols,  and  30  gm.  borax. 

The  bone-ash  of  the  cupel,  being  chiefly  calcium  phosphate, 
needs  much  borax  to  flux  its  lime  base.  The  material,  being  quite 
oxidized,  needs  just  enough  argols  to  give  the  .usual  2O-gm. 
button.  A  small  amount  of  silver  is  recovered,  which  is  added 
to  the  other  for  the  total  gold-silver  button,  and  the  result  in 
milligrams  multiplied  by  10.  The  buttons  are  then  parted  for 
gold.  Generally  four  to  six  results  are  made  from  as  many  tenths 
of  an  assay-ton.  The  buttons  may  be  parted  two  and  two,  so  that 
the  gold  is  weighed  in  one-fifth  of  an  assay-ton,  and  the  results 
must  check  one  another. 


XVII.— ASSAY  OF  CYANIDE  SOLUTIONS. 

Cyanide  solutions,  containing  silver  and  gold,  may  be  either  the 
solution  containing  the  precious  metals  before  extraction,  or  the 
solution  after  it  has  passed  through  the  zinc-boxes,  having  had 
its  values  mostly  removed  (barren  solution).  In  the  latter  case 
a  much  larger  portion  of  the  solution  must  be  taken  for  assay 
than  in  the  former. 

Of  the  numerous  methods  devised  we  give  two. 

FIRST  METHOD.  This  is  accurate  and  simple,  but  slower  than 
the  second.  It  consists  in  evaporating  to  dryness  upon  a  hot- 
plate, say  2  A.  T.  solution  in  a  lead-tray  made  by  turning  up  the 
edges  of  lead-foil  such  as  is  used  in  assaying.  This  is  formed  over 
a  wooden  block  and  may  be  2  by  2  by  ^4  m-  deep.  The  evapora- 
tion being  finished  the  tray  is  then  bent  into  a  compact  form  and 
directly  cupelled.  If,  however,  the  residue  is  considerable,  it  may 
be  advisable  to  scorify  the  lead  before  cupelling.  The  results  are 
to  be  reported  in  ounces  per  ton  of  solution. 

SECOND  METHOD.  This  method  is  more  rapid  than  the  first, 
and  can  be  used  with  a  larger  bulk  of  solution,  and  is  performed 
as  follows : 

Measure  into  a  No.  4  beaker  5  A.  T.  or  146  c.c.  of  the  solution. 
(If  the  solution  is  a  'barren  one,'  10  A.  T.  should  be  used.)  Add 
enough  KCy  to  the  solution  to  bring  it  up  to  0.5%,  KCy.*  Thus 
if  the  strength  is  0.2%  we  shall  need  to  bring  it  to  0.5%  the  addition 
of  0.3%  of  146  gm.,  or  0.43  gm.  of  the  salt.  Stir  into  the  solution 
10  c.c.  of  a  10%  solution  of  lead  acetate  (PbA)  slightly  acidified 
with  acetic  acid.  Add  from  the  point  of  a  spatula  about  0.4  gm. 
zinc  dust,  weighing  it  until  able  to  judge  of  the  quantity  needed. 
This  amount  of  zinc  is  amply  sufficient  to  precipitate  not  only  all 
the  lead,  but  also  the  gold  and  silver  in  the  solution.  Thus  we 
have : 


AUSTINS    FIRE    ASSAY.  75 

Zn  +  PbA  =ZnA  +  Pb 

Zn  +  2KAuCy2  =   K,ZnCy4  +  2Au 
Zn  +  2KAgCy2  =   K2ZnCy4  +  2Ag 

Stir  it  and  bring  to  boiling.  Add  15  c.c.  strong  HC1  to  dissolve 
the  excess  of  zinc.  Leave  the  solution  on  the  hot  plate  until  the 
zinc  is  completely  dissolved,  and  the  lead  has  gathered  into  a 
sponge.  As  soon  as  thus  dissolved,  remove  from  the  heat  to 
prevent  the  lead  from  being  at  all  dissolved.  Holding  the  lead 
with  a  flattened  glass  rod,  decant  the  solution.  Wash  the  sponge 
with  water,  squeeze  it  into  a  clot  with  the  fingers,  and  place  it 
upon  a  piece  of  lead-foil  weighing  5  to  6  gin.  Fold  the  foil  so 
as  to  leave  an  outlet  for  steam,  then  cupel  it.  The  resultant 
button  will  give  the  silver  and  gold  in  the  5  A,  T.  solution. 

*  To  determine  the  strength  of  the  KCy  solution  we  may  proceed  as 
follows :  Have  50  c.c.  burettes.  Fill  one  with  AgNOs  solution  consisting 
of  17  gm.  of  the  pure  salt  in  one  litre  of  water.  Fill  the  other  with  the 
solution  to  be  tested.  Measure  into  a  small  beaker,  13  gm.  of  the  KCy 
solution.  Run  in  cautiously  the  standard  AgNOs  solution  until  the  white 
precipitate,  which  has  appeared,  ceases  to  dissolve  where  the  contents  of 
the  beaker  has  been  stirred,  that  is,  when  only  a  faint  opalescense  remains. 
Ten  c.c.  of  the  standard  solution  equals  \%,  so  that  the  reading  is  to  be 
divided  by  10.  Thus  suppose  our  reading  was  14.5  c.c.,  then  we  will 
have  1.45%  KCy  present.  For  very  dilute  solutions  we  may  take  130  c.c., 
and  then  the  burette  reading  must  be  divided  by  100. 


XVIII.— ASSAY  OF   BASE-BULLION. 

Base-bullion,  or  work-lead,  is  the  product  of  the  silver-lead 
blast-furnace,  and  may  be  considered  as  an  unrefined  lead  con- 
taining upward  of  one  per  cent  of  silver  together  with  a  few 
per  cent,  at  most,  of  impurities,  as  arsenic,  antimony,  and  copper. 

The  sample  may  come  into  the  assayer  in  the  form  of  cylin- 
drical 'chips'  2  in.  long  by  l/s  in-  diam.  Since  two  of  these  pieces 
are  taken  from  each  ingot  or  bar,  there  would  be  800  of  them  for 
a  4OO-bar  carload,  and  amounting  in  quantity  to  a  quart.  A 
plumbago  crucible  is  made  red-hot  in  the  wind-furnace,  and  the 
chips  are  shot  into  it  by  means  of  a  scoop.  The  lead  quickly  melts 
down,  and  is  stirred  with  a  wooden  stick.  It  is  then  lifted  by 


J 


8 


FIG.  40.     BASE-BULLION   SAMPLE. 

the  tongs,  given  a  swirling  motion  to  complete  the  mixing,  and 
quickly  poured  into  a  mold  8*  in-  long  by  2.5  in.  wide,  where  it 
forms  a  bar  about  0.3  to  0.4  in.  thick.  This  bar  is  cut  longi- 
tudinally, by  means  of  a  chisel,  into  two,  each  half  being  num- 
bered; one  of  these  is  sent  to  the  refinery  and  one  retained  by 
the  works  producing  the  base-bullion.  Assuming  that  the  lower 
half  is  retained,  four  pieces  marked  in  as  i,  2,  3,  and  4,  each  of 
a  little  more  than  0.5  A.  T.  are  cut  from  the  bar.  They  represent 
a  fair  average  of  the  bar,  since  the  exterior  runs  a  little  higher 
than  the  central  portion  at  the  time  it  solidifies. 

In  case  of  an  impure  bullion,  each  sample  should  be  scorified 
with  extra  test-lead  and  a  little  borax  before  cupelling.  Where 
the  sample  is  sufficiently  pure,  direct  cupellation  is  satisfactory. 


AUSTIN  S    FIRE    ASSAY.  77 

Some  assayers  prefer  always  to  scorify  before  cupellation,  con- 
tending that  the  loss  is  less  in  scorifying  than  in  cupelling. 
Cupellation  should  proceed  with  the  formation  of  feather-litharge, 
and,  just  prior  to  brightening,  the  cupel  should  be  pushed  back  a 
little  to  the  hotter  part  of  the  muffle.  Where  the  buttons  are 
large  it  is  a  good  plan  to  have  cupels  set  just  behind  in  the  muffle 
to  cover  them  when  finished  as  in  the  assay  of  silver  bars.  The 
buttons  are  parted  as  in  ores. 


XIX.— ASSAY  OF  SILVER  BARS  OR  INGOTS. 

Silver  bars  are  assayed  to  determine  their  fineness,  or  parts 
in  a  thousand.  Thus  U.  S.  coin  silver  is  90%  or  900  fine,  the 
residue  being  copper.  Ordinary  ingots  or  bars  from  stamp-mills 
may  vary  from  250  to  900  fine  in  silver  and  gold,  the  impurity 
being  copper.  By  fine  silver  we  mean  pure  silver  bars  of  1000 
fine,  though  the  term  is  also  applied  to  bars  of  over  900  fine. 

Mill-bars  may  be  assayed  from  granulations  or  from  chips. 
To  make  granulations,  when  the  silver  has  been  melted  and 
refined  in  a  crucible,  and  just  before  it  is  poured  into  ingots, 
some  of  the  molten  metal  is  removed  by  means  of  a  heated  ladle 
or  scorifier,  and  poured  from  a  height  of  three  feet  into  a  bucket- 
ful of  water.  The  metal  is  broken  into  fragments  and  shot  of 
all  sizes  from  which  may  be  selected  particles  of  any  desired  size. 
Where  the  ingots  come  to  the  assayer  ready  cast,  he  takes  his 
sample  by  chips  cut  from  opposite  edges  of  the  bar,  the  chisel 
cutting  in  a  converging  way  into  it  to  take  out  a  wedge-shaped 
chip  of  from  2  to  5  gm.  These  are  placed  in  a  scorifying  dish 
in  the  muffle,  and  brought  to  a  just-visible  red  heat  in  order  to 
anneal  them.  They  are  then  hammered  out  upon  an  anvil,  again 
annealed,  and  put  through  rolls  (Fig.  41)  where  they  are  brought 
down  thin  enough  to  cut  readily  with  scissors.  Before  use  this 
sheet-metal  is  cleaned  by  rubbing  with  emery  cloth. 

The  operation  is  as  follows: 

TRIAL  ASSAY.  To  determine  approximately  the  fineness  of  the 
sample,  we  weigh  out  500  mg.  upon  a  button-balance,  selecting 
from  the  granulations  larger  pieces,  and  finally  the  finest  particles, 
to  obtain  the  exact  amount.  This  weighing  need  be  exact  but  to 
o.i  mg.  Where  the  sample  is  taken  from  a  rolled  sample,  we  may 
cut  out  a  longitudinal  strip  0.3  to  0.4  in.  wide  and  cut  this  trans- 
versely into  various  sizes,  down  to  mere  shavings,  in  quantity  more 
than  sufficient  for  an  exact  weighing.  This  is  wrapped  up  in  20  gm. 


AUSTINS     FIRE    ASSAY. 


79 


of  silver-free  lead-foil,  folding  it  in  both  directions  so  as  to  form 
a  cornet.  The  sample  is  dumped  into  it  from  the  pan  of  the 
balance,  and  the  foil  folded  up  ready  for  cupelling.  Cupellation 
is  performed  at  a  moderate  temperature  to  form  feather-litharge. 
Near  the  end,  the  cupel  is  pushed  back  into  the  muffle  and  finished 
at  a  higher  temperature.  A  hot  cupel  should  be  standing  ready 


FIG.  41.     METAL  ROLLS. 


in  the  muffle  and,  at  the  moment  of  blicking,  it  should  be  put  on 
the  other,  thus  excluding  air-currents  which  tend  to  cause  so 
large  a  button  to  sprout  or  spit.  This  is,  as  its  name  indicates, 


8o  AUSTIN'S   FIRE   ASSAY. 

the  formation  of  excretions  or  projections  upon  the  surface  of 
the  button  due  to  the  pressure  of  escaping  oxygen  which  has 
just  been  absorbed  by  the  molten  metal,  and  which,  upon  solidi- 
fication is  seeking  to  escape.  The  cupel  is  gradually  moved 
forward  to  the  front  of  the  muffle  and  as  soon  as  the  silver  gets 
solid,  is'  removed.  The  resultant  button  is  weighed,  and  the 
number  of  milligrams  multiplied  by  2  will  give  the  approximate 
fineness.  The  real  fineness  is  greater  than  this  by  10  to  15  fine. 
PREPARATION  OF  THE  PROOF-BUTTON.  The  button,  which  has 
been  carefully  weighed,  is  put  in  the  right-hand  pan  of  the  bal- 
ance, and  a  5oo-mg.  weight  in  the  left  one.  Enough  copper,  cut 
from  copper  foil,  is  added  with  the  silver  button  to  make  up  the 
weight  to  500  mg.  This  is  transferred  from  the  pan  to  a  cornet 
of  lead-foil  in  the  manner  already  indicated,  the  quantity  of  foil 
to  be  taken  being  determined  from  the  table  herewith. 


WEIGHT  OF  LEAD 
FINENESS 

IN  GRAMS 


950  to  looo 
800  to  950 
600  to  800 
600  and  less 


5 
10 

15 
20 


Thus,  suppose  we  had  found  by  the  trial-assay  a  button  of  312 
mg.  equal  to  an  approximate  fineness  of  624,  we  would  see  from 
the  table  that  15  gm.  sheet-lead  would  be  enough.  Two  other 
portions  of  the  sample  are  taken,  each  of  500  mg.,  and  wrapped  in 
the  same  quantity  of  lead-foil  as  the  proof.  The  three  are 
cupelled  abreast  in  the  muffle  as  in  Fig.  42,  taking  care  to  cupel 
them  all  at  the  same  temperature  and  in  such  a  way  as  to  form 
feather-litharge.  The  cupellation  loss  of  each  of  these  may  be 
considered  as  being  the  same.  The  fineness  is,  therefore,  the  sum 
of  the  two  outer  buttons  plus  twice  the  loss  in  cupelling.  Thus 
suppose  the  results  are  as  given  in  Fig.  42.  Then, 

3i8  +  3i7  +  (2  X  6)  ==  647, 
which  will  be  the  actual  fineness.    It  will  be  noticed  that  the  proof 


AUSTIN'S   FIRE  ASSAY. 


8  1 


is  made  up  to  imitate  the  sample.  Strictly  speaking,  5  to  10  mg. 
of  silver  should  be  added  to  the  silver  button  to  allow  for  the 
cupellation-loss,  or  we  may  take  the  same  weight  (in  this  case 
512  -f-  6  =  518  mg.)  of  pure  silver-  foil.  The  rest  of  the  proof 


3/7 


FIG.  42.    CUPELLING  FINE  SILVER. 

is  assumed  to  be  copper ;  but  its  exact  amount  may  be  determined 
by  assay  in  the  wet  way.  The  duplicates  should  not  vary  from 
one  another  more  than  one  milligram. 

The  gold  is  determined  by  parting  the  duplicates  together — 
the  weight  in  milligrams  being  the  fineness,  and  this  may  be 
checked  by  parting  the  proof-button. 


XX.— ASSAY  OF  BLISTER  OR  PIG-COPPER 
CONTAINING  SILVER  AND  GOLD. 

The  losses  of  silver  in  attempting  to  assay  copper  matte  and 
blister-copper  by  scorification  are  quite  large,  and  it  is  accordingly 
best  to  remove  the  copper  by  a  wet  method  before  fire-treatment. 

The  Whitehead  method  of  blister-copper  assay  is  as  follows : 
One-half  of  an  assay-ton  of  the  granulations  is  introduced  into  a 
No.  4  beaker  with  50  c.c.  water.  To  this  we  add  25  c.c.  nitric 
acid  of  1.42  sp.  gr.  and,  when  action  has  nearly  ceased,  another 
25  c.c.  is  put  in.  The  solution  is  allowed  to  stand  in  a  warm 
place  until  the  red  fumes  have  disappeared,  after  which  it  is 
diluted  to  300  c.c.  The  copper  and  silver  have  gone  into  solution 
thus : 

3  Cu  +  8HN03  =  3Cu  (N03)2  +  4H,O  +  2NO 
3  Ag  +  4HN03  =  3AgN03  +  2  H2O  +  NO 
while  the  gold  remains  as  an  undissolved  residue. 

We  now  run  into  the  liquid,  drop  by  drop,  a  10%  solution  of 
common  salt  (NaCl)  in  slight  excess  to  precipitate  the  silver  as 
chloride. 

AgNO3  +  NaCl  =  AgCl  +  NaNO3. 

The  solution  is  well  stirred,  and  10  c.c.  of  a  10%  solution  of 
lead  acetate  added  and,  still  stirring,  2  c.c.  of  sulphuric  acid 
(i  part  acid  to  I  part  water),  after  which  it  is  allowed  to  stand 
until  settled.  The  sulphuric  acid  precipitates  the  lead  thus: 

PbA  +  H2SO4  =  PbSO4  +  H2A, 

and  the  lead  sulphate,  in  falling,  carries  down  the  gold  mechani- 
cally. Finally  the  solution  is  filtered  off,  and  the  precipitate 
washed  on  the  filter  to  remove  copper  salts.  The  filter  paper  is 
removed,  folded  over  the  precipitate,  placed  in  a  scorifying  dish 
on  top  of  10  gm.  of  test-lead,  and  then  gradually  burned  at  the 
front  of  the  muffle.  A  cover  of  10  gm.  more  of  test-lead,  and 


AUSTIN'S   FIRE  ASSAY.  83 

I  gm.  of  borax  is  added,  and  the  whole  started  to  scorifying. 
When  scorified  the  lead  buttons,  weighing  6  to  8  gm.,  are  cupelled 
with  the  formation  of  feather-litharge  and  the  gold-silver  button 
weighed  and  parted.  The  work  should  be  done  in  duplicate,  and 
the  results  should  agree  almost  exactly. 

The  method  may  also  be  used  for  the  determination  of  gold 
and  silver  in  base  ores,  such  as  gray  copper  or  arsenical  pyrite, 
or  of  high-grade  copper  matte. 


XXI.— ASSAY  OF  GOLD  BULLION. 

The  sample  is  generally  taken  by  two  chips  from  opposite  edges 
of  the  bar  as  in  the  silver  assay.  For  a  small  bar,  as  little  as 
100  mg.,  and  for  a  large  one,  500  mg.  may  be  taken. 

The  base  metals  are  first  removed  by  cupellation,  wrapping  the 
500  mg.  in  10  gm.  of  lead-foil.  There  remains  behind  the  pure 
gold-silver  button,  which  is  weighed  to  determine  the  fineness  of 
gold  and  silver  together. 

The  silver  is  removed  from  the  gold  by  inquartation,  taking 
500  mg.  of  the  original  sample,  to  which  is  added  three  times  the 
weight  of  the  pure  silver-gold  button.  This  is  cupelled  with  5  gm. 
of  lead-foil,  since  it  is  not  desired  to  remove  all  the  copper.  If  no 
copper  is  present,  10  mg.  of  pure  copper  should  be  added.  The 
resultant  button  is  weighed,  then  flattened  out  under  the  hammer. 
It  is  annealed  and  then  passed  through  the  rolls  (Fig.  43),  which 
draws  it  out  into  a  piece  about  four  inches  in  length.  When  cold 
it  is  rolled  around  a  lead  pencil  into  a  spiral  coil,  and  parted  in  a 
porcelain  capsule.  It  is  boiled  first  for  ten  minutes  with  I  to  I 
acid  (1.20  sp.  gr.),  then  with  stronger  acid  of  7  parts  acid  to  4 
of  water  (1.30  sp.  gr.)  for  the  same  period  for  the  removal  of  the 
last  traces  of  silver.  The  acids  are  decanted,  and  the  residue, 
which  is  quite  coherent,  washed  with  pure  hot  distilled  water. 
The  gold  is  now  dried,  ignited  at  a  low-red  heat,  let  cool,  and 
weighed.  This  weight,  subtracted  from  the  first  one,  gives  the 
contained  silver.  The  results,  as  before,  are  doubled  to  express 
the  fineness. 

This  method  is  sufficiently  exact  for  ordinary  commercial 
assays.  For  the  more  precise  methods  of  the  government  assay 
offices,  see  Furman's  'Manual  of  Practical  Assaying/  page  246. 


XXII.— ASSAY  OF  ORES  CONTAINING  METALLICS. 

Ores  containing  native  gold,  silver,  or  copper,  and  by.-products 
of  metallurgical  operations  having  metallic  particles,  prills,  or 
scales,  are  assayed  as  follows : 

The  ore  is  pulverized  upon  the  grinding  plate,  the  metallics 
flattening  out  into  scales  which  will  not  go  through  the  screen. 
We  accordingly  have  the  screened  material,  which  must  be  thor- 
oughly mixed  before  assaying  it,  and  the  metallics  remaining  on 
the  screen.  Both  the  fines  and  the  metallics  are  weighed  sep- 
arately. 

Either  one  of  the  following  methods  may  now  be  followed : 

Method  i.  In  the  total  quantity,  determine  the  percentage  of 
metallics.  Where  a  half-assay-ton  is  to  be  taken,  figure  this  per- 
centage in  grams.  Place  that  quantity  on  the  pan  of  the  pulp- 
balance,  and  make  up  to  the  half-ton  with  fines.  We  thus  assay 
the  whole  sample  in  the  proper  relative  proportions,  and  easily 
estimate  the  average  result. 

Method  2.  Where  the  metallies  are  irregular  in  value  and 
small  in  quantity,  containing,  say,  silver,  copper,  and  gold  par- 
ticles, it  is  better  to  assay  the  whole  and  determine  the  value  in 
ounces  per  ton.  Thus  if  the  metallics  weigh  7.3  gm.,  giving  in 
that  quantity  156  mg.,  we  would  have  29.166  X  156  —  623.3  oz- 

7-3 

per  ton.  Let  us  say  that  the  fines  weighed  720  gm.  and  assayed 
1 15.0  oz.  per  ton.    Then,  to  get  the  average  value,  we  would  have : 

720.0  X  115.0  =  82800.0 
7.3  X  623.3  =    4550-1 


727.3       into         87350.1  =  1 20. i  oz.  per  ton. 
Where  the  fines  preponderate   so  greatly,   we   may   expect  this 
result  to  be  but  little  more  than  the  lower  figure  of  1 15  oz.  per  ton. 


86  AUSTIN'S   FIRE  ASSAY. 

_%:"- 

An  example  of  an  ore  carrying  native  copper  would  be  as 
follows : 

The  whole  sample,  consisting  of  pieces  1.25  in.  diam.  and 
smaller,  weighed  142  oz.  and  contained  2.75  oz.  of  metallics  which 
were  picked  out,  freed  from  adherent  gangue  by  hammering 
them,  and  were,  by  the  Whitehead  process  (Chapter  XX)  found 
to  contain  54  oz.  Ag  per  ton. 

The  residue  was  regularly  sampled,  but,  when  finally  ground 
to  pass  an  8o-mesh  sieve,  gave  0.8  gm.  metallics  and  124  gm.  fines. 

On  determination  the  fines  gave  0.3  oz.  Ag  per  ton  and  the 
metallics  were  assumed  to  have  the  same  value  as  in  the  first 
metallics  (in  the  2.75  oz.). 

Hence  we  have : 

124.0  X    0.3  =  37.2 
0.8  X  54-0  =  43-2 


124.8     into         80.4  =  0.64  oz.  Ag  per  ton. 
Then,  referring  to  the  whole  sample,  we  have: 
139.25  X    0.64=    89.12 
2.75  X  54-0    =  148.50 


142.00  into  237.62  =  1.67  oz.  Ag  per  ton. 


XXIII.— THE   LEAD   ASSAY. 

Any   lead-bearing  ore  may  be  assayed   for  lead,  the  process 
being  a  simple  reduction  fusion,  similar  to  the  nail-assay.    Where 
the  ore  contains  much  pyrite  it  may  be  first  roasted,  as  described 
in  Chapter  XIV,  using  10.  gm.  for  the  roast.     Ores  are  assayed 
for  lead,  using  the  following  charge : 
10  gm.  ore, 
20  gm.  soda, 

5  gm.  argols, 

4  to  15  gm.  borax  fused  as  a  cover),  and 
i  to  5  ten-penny  nails. 

This  charge,  in  a  lo-gm.  crucible,  is  melted  down  gradually  so 
as  to  avoid  boiling  over  and  is  finished  at  a  high  heat.  As  soon 
as  it  is  in  quiet  fusion  it  is  poured.  It  should  not  be  kept  long 
in  the  furnace  as  one  may  do  in  a  silver  assay,  but  must  be 
poured  as  soon  as  reduction  is  complete. 

Drops  of  lead,  still  clinging  to  the  nails  as  they  are  withdrawn 
from  the  crucible  before  pouring,  indicate  incomplete  reduction. 
The  melt  is  poured  into  a  conical  mold,  and,  as  soon  as  cool,  the 
lead  button  at  the  point  of  the  cone  is  removed  and  hammered  to 
remove  adherent  slag.  The  button  is  weighed  to  the  nearest  tenth 
of  one  per  cent.  The  average  of  duplicates  may  be  taken.  They 
should  agree  to  within  0.5  to  \%.  This  fire  method  of  lead  assay- 
ing gives  results  a  little  lower  than  the  true  one,  unless  there  are 
impurities  as  copper  or  antimony  in  the  ore,  which  being  reduced, 
go  into  the  lead  button.  Its  execution  requires  the  best  skill  of 
the  assayej,  and  depends  on  care  and  skill  in  the  fire-work.  The 
slag  from  such  an  assay  pours  thicker,  or  is  stiffer,  than  from  a 
silver  assay.  The  same  precautions  in  regard  to  fluxing  should  be 
taken  as  in  a  nail-assay  for  silver.  The  soda  fluxes  the  silica  and 
dissolves  any  matte  which  may  be  formed.  The  quantity  of  borax 
must  be  varied  according  to  the  amount  of  bases  present.  Lead, 


AUSTIN'S  FIRE  ASSAY. 

since  it  is  reduced,  does  not  enter  the  slag  and  hence  is  not  a  base 
that  must  be  taken  care  of  by  borax.  The  nails  are  put  in  accord- 
ing to  the  needs  of  the  sulphur,  the  maximum  number  of  five 
being  used  for  a  pyrite  or  blende  ore,  while  a  single  one  only 
would  be  put  in  for  an  oxidized  ore. 

The  fluxes  mentioned  above  are  often  put  in  in  mixture,  the 
reducing  mixture  (Chapter  VIII)  having  borax  added  to  it  as 
follows : 

20  parts  monocarbonate  of  soda, 
5       "      flour, 
5       "      borax. 
They  are  thoroughly  mixed  and  put  through  a  lo-mesh  screen. 

When  thus  used,  the  charging  of  the  crucible  is  quite  simple. 
The  measured  amount,  say  30  gm.,  is  put  in,  then  10  gm.  of  the 
ore,  and  both  mixed  with  a  Spatula.  A  cover  of  salt,  one-fourth 
inch  deep,  follows,  then  the  needed  number  of  nails.  Additional 
borax  may  also  have  to  be  added. 

Some  assayers  claim  that  in  the  fire-work  the  assay  should  be 
melted  rapidly  and  brought  to  a  high  heat  before  pouring.  It  is 
well  for  the  assayer  to  follow  the  method  that  gives  him  the  best 
results. 


4950 


179709 


